At the end of the day most are looking at the songs, are they fun or not? ... If part of the original $60 went towards the dreamscape, venues, and avatars then I would be fine with an additional $10-30 export fee.
Certainly people would pay to play the songs in the main platform. I've spoken with a couple of players I know that chose not to buy the game because it was walled off. If not for my wife I too would have been in that group.
The game play itself - avatars, dreamscapes, and even harmonies isn't enough to sell the game to the casual player. People will always look for added value. I believe that is one factor that has people migrating more and more to the Rock Band Platform versus other music games.