I think many hubs is probably the right answer, but it depends on the goal.
If the goal is to feed people into MIRI, or generally advance some single community organization as far and as quickly as possible, then the benefits of centralization are well known and hard to overstate. Similarly, if the goal is to have a social environment for rationalists, where one’s friendships and broader social circles mostly involve other rationalists, then one hub is also clearly the best goal.
However, if the goal is to spread awareness of rationality, raise the sanity waterline generally, grow the movement, or find new ideas and fruitful applications, then many smaller hubs are a vastly better option. All of these things are encouraged by putting rationalists in differing environments, where they contact a wider variety of people, ideas, and problems.
It seems to me that the stated goals of the community are the latter, and so the “right” answer is obvious. We should be encouraging people to find and befriend like-minded people in their local area, and introduce the ideas to their potential rationalists. Have many friends, rationalist and not. Figure out how to talk about rationality’s greatest insights quickly and concisely, so you can give people that ah-ha moment and get them hooked. Etc.
To be clear, Berkeley people have a community too, and I don’t think we should tell them they have an obligation to move out and evangelize, like early Christian apostles. But I think we should stop encouraging people to move there and if someone is fed up and wants to leave the Bay, they should be bid a happy farewell.
I also think centralization in Berkeley causes damaging PR problems, and that this is a significant point in favor of many hubs. But that’s in a comment because it deserves separate attention and may invite passionate response.
A story:
I was rationalist adjacent for a long time, but I never spent much time on LessWrong.com or felt like a member of The Rationalist Community. Partly, I was intimidated by the posters here and had (still have) some pretty core philosophical disagreements. But a lot of it was because every time I came here or to any other rationalist’s blog, I would see a bunch of obliquely-referenced personal drama and inside baseball stuff about the community in Berkeley.
Eventually, Scott’s SSC Meetup Everywhere post convinced me to check out my nearest group. I met some really cool people, who were a lot more normal and friendly than I’d been led to expect. Now I run a meetup in my local city, and do some outreach. I’m pretty sure that I’m the kind of person who’s a really good fit for joining the cause, the kind we want more of. But I didn’t know it from the online community.
Rationality is already pretty weird, and rationalists tend to be quirky people. But my experience visiting and reading about Berkeley is that merely living there encourages one to spend freely and frivolously on weirdness. (I mean this of the city generally; not the rationality community there, whom I’ve never met and know only by reputation.) If you want to raise a kid and live together in a single house with your 5 best friend-and-sexual-partners, it’s no business of mine. For all I know, I’ll be thanking you in 100 years for discovering the optimal living arrangement. But it’s a very Berkeley thing, which leads to very Berkeley problems, which all tend to be viewed pretty negatively by the outside world.
To the extent that rationalists centralize in Berkeley, public forums become largely Berkeley-specific forums, in which weird, Berkeley-specific problems (social drama, weird living arrangements, etc.) are a normal and noteworthy topic of discussion. Even if there’s not frontpage posts, it leaks out into the public through comments and offhand mentions in discussions about rationality. And that weirdness and drama leakage can really turn people off from the movement who aren’t in Berkeley, even when they’re really attracted to rationality as a practice and movement.