On Mightygodking's blog he made a point that since Dr. Strange is/was Sorcerer Supreme, he's responsible for the mystical/pan-dimensional defense of the Universe. While he would like for the Earth to remain peaceful, he's not duty bound to protect it from Skrulls, or Apocalypse or whatever enemy that doesn't want to destroy the Earth/threaten the mystical fabric of the universe. Stranger should've never been in the illuminati because most things that would interest them are not on Strange's radar.
And Xavier hasn't been "xavier" for years. Morrison on a meta-level explained why his philosophy (and magnetos as well)was no longer relvant. Coupled with the fact that all Xavier's been doing for the past 4 or 5 years is paying for his sins makes him pretty inefficient.
Plus I guess the fact Magneto will kill anyone for the sake of mutants makes him more efficient in the long run of "protection".