A couple of us have already proved this theory of yours wrong. I have never been to a grateful dead show and they're one of my favorite bands. I'm certainly not in it for the "tribe".
You really haven't. In fact, looking at this board, the social benefits of identifying with a particular "tribe" is constantly reinforced. The behavior and postings patterns of the members of this board actually reinforce my theory. Which isn't really my theory.
And of course not every single person is in it for the same precise reason. we're talking in global sense (though I think you're far more in the tribe than you think -- you didn't see the Dead because you're the wrong generation). And jam bands' enduring popularity has very much to do with its tribal aspects. Same with most kinds of pop music. It's not a bad thing.
But people rail against it because we all want to think we are unique little snowflakes. Which isn't hurting anybody, so go on ahead.