Superman, Q, Phoenix and Dark Phoenix -- they're all aspects of the same thing, and what it comes down to is conflict. If you have an invulnerable or omnipotent character, you lose any meaningful conflict; if the super-character is on your side, there's no risk of losing, and if it's on the other side, there's no chance of winning.

This is Writing 101, guys! Even I know that much, and I'm not a writer. Why do so many people forget how it works?