cuz they want to force everyone to play the way they do. just inconsiderate and elitest, ignore 'em
Ahh.... there are few things I enjoy more on a forum than seeing someone figuratively haul off and punch himself in the face.....
To the topic - the real point, to me at least, of having rival factions is for the story possibilities. It's just much more engaging if my characters have to make choices that matter - that's part of what takes it beyond just another game to be cruised through, then set aside. TES games aren't meant to be rushed through pell-mell - the TES world is there to be experienced and savored. And part of that is making each character, and each character's story, unique. And part of that is having the character come to points at which s/he has to make a choice between one path and another. If both paths are equally possible, then there's no meaningful choice, and without meaningful choices and consequences to those choices, characters are less distinct and the game is less fulfilling.
It's not so much that the character should be blocked off from one faction if s/he joins a rival faction - it's that the dynamics within and between the factions have to make sense. IF it makes sense that membership in one faction precludes membership in another, then that's the way it should be. IF it makes sense that one could join both, then that's the way it should be. IF there's some way that one could join two rival factions and get away with it, that's the way it should be.
Beyond that, there's also an issue of time and effort - in much the same way as a single character simply isn't going to be able to invest enough time and effort to successfully master multiple skills, a single character shouldn't be able to invest sufficient time and effort to lead multiple factions. I hope that the quests are set up so that it will take just about the entire length of a normal playthrough to climb all the way to the top of a faction. And doing so should be quite difficult AND quite rewarding.
That's my view on the matter at least.......