LOL y'all Cassie haterz are just JELLY.
She's one of the greatest X-villains of all time. I seem to remember her winning that poll a couple years back... the Best X-villain survivor thing.
Anyways, I agree that with Charles gone she doesn't really have a hook for returning yet, but damnit, his precious muties still live on. And as long as they're alive, trying to carry out his dreams, there should be someone around who's so diametrically opposed to seeing anything Charles cares about come to fruition to eff things up and make life miserable for them.
Oh and let's not forget about the fact that she makes Beast crap his pants whenever her name is even mentioned. His reaction to her return alone would be worth it, provided the writers know what they're doing with the two of them.