Game balance is all about context, and so it is pretty impossible to balance a game perfectly. I think limelight is an idea which works much better. Do all characters have a chance to shine and be cool? Are the players' gaming needs being met? Of course, you don't want obviously sub-standard choices because that's not fun, and can lead to frustration. It's one of the reasons I really appreciate FATE. Players can make what they want, be happy, and trust that it will work in the game.
The old Buffy game worked well too, and did a great job of being able to mix "super heroes" like Buffy, with characters like Xander. Both choices were viable and fun to play.
Some of the recent super hero games have tackled this issue too. How do you balance Super Man with Green Arrow? Older games said, "Don't do this." Newer ones have realized that it works, if they both get a chance to do their thing regularly. Super Man takes on the task that needs massive strength, for example, while Green Arrow is there to make that impossible shot which will bring down the alien ship. Super Man's problems and enemies tend to be more earth shaking, while Green Arrow deals with more street level problems. Obviously, Super Man is way tougher than Green Arrow, but in a well designed game, they can work together, just like in the comics.