Good point - I didn't mean that a big group of mutants IS a must, but the studio certainly believes it to be.
In a way, adding more mutants does have an appeal, because if you love Jean Grey and I love Storm and both are in the movie, we're both likely to want to see it.  If a movie only stars Magneto and I despise Magneto, they've potentially lost my ticket money.  It's just a shame that mutants get stuffed in based on visual appeal without regard to making them an important or interesting character (pretty much the entirety of Magneto's army in X3, and quite a lot of the young recruits in XMFC).