It seems the only reason for a 2.5 league is to gently introduce people into competition. Having the shenanigans associated w/ going for state or nationals is just going to muddle things up. The reward for doing well in 2.5 should be escaping the beginner's league.
I'd also argue that on the other end, with 5.0 and 5.5s, you're getting to the point where most of those players have competed in college or bigger stages... If I were that good, going for a USTA trophy seems a big step backwards...
For the rest of us hackers who never got to live the dream, nationals is obviously a very big carrot.
Of course, I have to concede Tom's point about travelling w/ buds