Do they really want the same thing? It's hard to tell when we're also offering other tasty treats. Prostitutes will do things for cocaine that they wouldn't do otherwise, and pretend to enjoy themselves.

Maybe they would, maybe they wouldn't. But if rationalism doesn't at least offer something comparable to other options, many people won't even try.

As for Aumann, Robin Hanson points out that we'd often do better to update toward "average beliefs" than the beliefs of our chosen in-group. So it appears I can already maximize my "Aumann benefit" by conversing with random strangers. It seems to me that the benefits of a rationalist group are precisely the opposite: Discover good arguments and important data that we were previously unaware of, that we find convincing regardless of source. If we value our own mere opinions too highly, we've already lost.

Robin's argument in that link seems to be that taking pleasure in disagreement with average beliefs is, all else equal, a bad thing; it's certainly not an argument in favor of updating toward average beliefs. Aumann agreement only strictly applies to ideal rationalists with shared assumptions, but as a rule of thumb one should update toward other agents' beliefs based on the demonstrated rationality of their belief-forming process.