Seems to me that if they aren't learning how to use their powers at less than full force, they haven't learned how to be efficient or "do more with less." Evil Scott might not know how to use his optic blasts as effectively or creatively as our Scott, if he's used to being a cannon, and if Angel is used to getting the drop on his victims from above, that trick won't work once a canny opponent learns it. To cite two possible examples.

Lastly, these otherworlders are clearly used to having Magneto as their combination team leader and big gun. If he's taken out, how well can they fight without him?