Originally Posted by Caveat Emperor
American sports are fundamentally different than just about everywhere else because we're so much larger of a nation than anyone else. Remember -- the entire Premier League fits into an area that size of Alabama. We'll never reach a point in America where there are 5 teams in New York (ala Arsenal, West Ham, Fulham, Tottenham, and Chelsea) or 2 teams in Baltimore (Man U, Man City). There isn't enough money or attention to go around.
I suspect the future for MLS is the same as it is for most American sports -- ~30 teams in the top metro areas (good news for Cincinnati / Dayton's eventual chances) and another ~15-50 permanent B/C level minor league squads elsewhere.
I don't think the only reason you have a promotion/relegation system is due to proximity though althought it does help for sure. The biggest factor is that there aren't as many sports and the attention is more focused like you mentioned. I tend to feel that soccer is such a global game, that its growth here would overcome more American conventions. Basically every league in the world has that type of system and there are actually a lot of benefits to growth. Why would a B level team in Cincinnati put a lot of money into salaries and facilities if they are always going to be a B level team?
Like I said I don't think this is going to happen anytime real soon and is contingent upon continued growth of the sport. However if interest does continue to increase and the league continues to grow, I can see it as a real possiblity at some point in the future.
Someone also mentioned baseball, but I think that is somewhat apples and oranges since it is the farm system not a bunch of independent teams.