I seem to recall you saying things to the effect of Usopp being the emotional core of the crew or being a big part of meeting the emotional needs of the crew (the precise wording escapes me). Do you have any meta on that/have you posted meta on that before? I'd love to have a link to read it!
Sorry for the lateness of my reply, Anon! I wonder if I've ever said that Usopp was the emotional core of the crew - it sounds a bit too definite and exclusive to my ears - but the second wording does sound like something I could have said, yeah. I can't think of any specific meta post I've made to that effect, though. I'll put down some rambling thoughts here instead, but that's all they'll be - obviously terribly subjective, and I'm not going to include any manga pics or the like from canon to make any kind of coherent case.
Many thanks to tresdem and silverwolfcc for listening to me on Plurk about this, helping me sort out my thoughts and contributing! (Everyone should read tresdem's ongoing One Piece meta blog we-are!.)
Before anything else, I need to point out that with a canon as long as One Piece and a group that's as large as the Strawhats - nine is very small for a pirate crew but for the group of central characters, it's pretty big - it's hard to make any definitive type of statements about how the groups dynamics work in canon. You can make generalised statements such as "This character tends to take on this role" and "this type of reaction will often be left to this character", but there will still be instances where the dynamics play out differently. An obvious example is when the crew is split up into smaller groups, as invariably happens during an arc. If there's something that needs to be pointed out in order to push the plot forward/provide for exposition/drive home a point, and the character who would usually do it isn't around, it's good if there's another character around who can do the honours while also being in character.
That being said, some characters are more inflexible than others. Brook is both easily - comically frightened and genuinely badass - but you generally won't find him as the down-to-earth voice of common sense. Chopper will not be impervious or hard to fool if someone plays to his sympathies, nor will he ever be poker-faced; Robin on the other hand will virtually never have a comically exaggerated reaction shot. And Luffy is... well, always Luffy, pretty much. Even when he surprises us, it's generally not in a way that could be replaced with anyone else, as his position as well as his personality always makes him the most singular crew member. It's remarkable how well Oda's succeeded at keeping him unique. (Although I think it helps that you often get the sense that both us readers and Luffy's crew are very invested in Luffy's uniqueness.)
Okay. Usopp is often called the most 'normal' character of the Strawhats; and I do think it's true in some respects. But we shouldn't forget that he also fills the comic relief slot more often than other characters (expect perhaps Luffy himself??) which pushes him to his own comical OTT reactions and quirks. Pre-timeskip, I think it might actually be Sanji who shows the most well-rounded, three-dimensional personality of the Strawhats, who's socially adept and very flexible in the way he reacts and how he interacts with others notwithstanding his hang-ups about women and habitual squabbling with Zoro. (Post-timeskip Sanji is sadly often more of a caricature of his old self.) In other respects such as not being (as) superhuman, having a common sense avoidance of dangers etc both Nami and Usopp are the 'normal' ones.
Usopp, Sanji and Nami are all relatively social and emotionally adept - they're quite good at reaching out to new people and forming bonds with them. (Well, not as much early on for Nami - but post-Arlong Park she makes up for it.) So is Luffy, of course - but Luffy has his distinct Luffyness that often baffles new people even as they're amused and impressed by him. It's up to the rest of the crew to provide more normal reactions to anchor new people with - and Usopp, Sanji and Nami tend to be the best at that.
Nami tends to be less chill about things than Usopp, though - she has a more driven and in-control personality and I think she feels the responsibility of being the navigator and Chief of Budget keenly, which limits her patience with what she sees as idiocy/overly silliness. Plus, she's also constrained by the creator's views on what's acceptable for the main girl to do. She does have a mischievous side but we don't see it in play too often anymore.
I'll go through Strawhats from Luffy to Chopper individually with what I think they get out from Usopp being there. Robin, Franky and Brook are a bit harder to sum up and in the interest of getting this rambling post to an end I'm afraid I'll leave them out, at least for now.
For Luffy, Usopp is the guy most likely to geek out and have fun over the same kind of things with him, to be ready for pranks and goofiness. There's also Chopper and Brook, and they can be silly without Usopp too - but I get the feeling that if Usopp hadn't been there in the first place, Chopper wouldn't have been drawn into shenanigans with Luffy the same way - he needed the two of them there to really express and develop the funloving side of his own personality. Luffy didn't have the kind of need Chopper did, he was always like that; but before Usopp arrived, he was more likely to just be goofy on his own. Things are more fun for Luffy when Usopp is around. It probably helps that they share the same kinds of ideals when it comes to piracy, even as Usopp has a hard time to truly grasp the putting-your-life-on-the-line thing on an everyday, always-there level the way Luffy does.
Zoro is always a little hard for me to parse but the way I read him is that for Zoro, Usopp is a good friend and comrade who accepts him the way he is - he may grumble about him and not always understand him too well - since they have very different personalities - but he doesn't want him to change and doesn't demand anything from him other than being a handy shield when there's danger. Just as Zoro needs Luffy to expect great things from him and demand the near-impossible so that Zoro can push himself to new heights, so I think he appreciates having someone who he can have a more chill dynamic with.
For Nami, Usopp is the brother-in-arms when it comes to freaking out and running away and speaking up for the virtues of self-preservation and not automatically taking the most dangerous way possible. (Chopper does often join in, but Chopper is more easily influenced by Luffy's pronouncements, although it's a bit of a sliding scale where Nami is at one extreme of the scale and Luffy at the other. Usopp gets sparkly-eyed over Cool Stuff too, but he's more likely to take Nami's part about the dangers than the others are.) Nami's come to Usopp's aid probably as often as the other way around, and they have a strong bond of mutual trust and understanding.
For Sanji, Usopp is a kind of younger brother and good friend and the one he's most likely to relax around and get casually physical with. They can exasperate each other but it soon blows over and he's not someone who Sanji feels like he needs to prove his masculinity to: he's both protective towards him and understands his pride, none of which stops him from chewing him out or kicking him in the head when he's exasperated. But Sanji exasperates Usopp too, and they're still great bros. Usopp is pretty much Sanji's best friend and the other way around - or at least, Sanji is Usopp's Best Friend Who Isn't Luffy.
We've already covered Chopper above: Usopp made it easier for Chopper to get into the Strawhats and let out his funloving side - if it had just been Luffy alone Chopper might have been too baffled or even intimidated. But Usopp is a kind of youngest-big brother character Chopper enjoys looking up to (even if it's not always earned! ^_^) and play with, and who shows cleraly he enjoys spending time with Chopper as well.
In the end, I would say that Usopp is essential to the inner dynamics of the Strawhats because he's good at reacting to what's going on without being all cool and tough or Luffyesque about it, may have it easier to express the soft and squishy emotions than Nami sometimes does and is less gullible and less ignorant of how the world is than Chopper. In spite of his lying he's actually quite authentic in his feelings and he's not a combative testesterony guy. And he's talkative and outgoing and isn't really afraid of trying to talk to people even when it's hard to find the right words to say. He's a pretty direct kind of person.
(Speaking as someone who wrote a fic where Usopp is no longer around, in purely practical terms that could slow down the writing especially for crowd scenes. There were things that needed to be said both for plotting and characterisation that I felt Usopp would normally have said, and now they needed to be given to other people - mostly Nami, Sanji and Chopper as it happened - who might not usually have said it as naturally as he would have done.)
You could say that in one sense, Luffy is if anyone the emotional core of the Strawhats - he's the motivator, the one that's brought them all together and who they all so strongly believe in. Luffy has a frontal attack style when it comes to making new friendships, while Usopp is more of a slide-through-the-cracks kind of person. However, Usopp's own qualities make him one of the essential ways for the crew to keep together and function well as a social and emotional unit.