Personally I think that if you've levelled a character to 90 you should be allowed to roll a pre-made whenever you want. I agree that the current solution is bad because it basically destroys the levelling experience for new players. There's little challenge before level 85, and you've already out-levelled most zones before you can even get half way through them. If you happen to try a dungeon you might as well skip a couple of zones because you will come out of it several levels higher. This makes the levelling experience slightly less painful (but still painful) for existing players who don't like to level, but it really screws with first-time players.
I should also mention the huge skill gap between players in low-level dungeons. There are always one or two first-timers in there who are still learning their class and have no idea as to how group dynamics should work in WoW. Then you get the abusive jerk who is levelling his 20th character and just wants to get through the dungeon as quickly as possible. That guy usually spends a good 15 minutes ranting at the first-timers before finally rage quitting or being vote-kicked. It's not a happy experience for anyone involved. I think that's another case in favor of premade characters.