Yeah, Claremont was once asked about that, and he said that when Magneto and Xavier were, at differing times, members of the X-men and actually went on missions with them, he always had to de-power them to make the action more suspenseful. So he would make them less powerful or he'd come up with a plot device to take them out quickly. Otherwise, he said, "all their battles would be over in a single panel".
I'm not crazy about it, but...eh? What can you do?