In Janssen's interview over at MTV Overdrive she states that Jean is with the Brotherhood, not necessarily because she wants to be with them (or that she was persuaded to be with them), but that she really has no other place to go--and is just there . . . which is why you see her always standing off to the side, rather than side-by-side with Magneto. She is simpy there because she is confused and really feels she has no other place to go--not because Magneto is specifically using her for anything.
Regardless of Jean's motivations, I see the thread maker's point. If Magneto is so powerful, why even bother with Jean? Why even bother with anyone else for that matter, since he seems so easily capable of solving matters on his own (he certainly has the power to do it). Even though Magneto is powerful enough on his own, he is still power-hungry, and although it may be risky, why not go the extra mile and harbor the most powerful mutant in the world among your Brotherhood?