as someone mentioned, to cast such a well known singer for a secondary role like the ones Fox introduced on FC, and with possible future characters as Dazzler, Sunfire or Polaris, they shouldnt cast someone as famous, because that right cast would become more of an event itself, rather than letting the audience connect with the new character. The audience would just see that famous person, not the character.
If Kimberg and Singer have decided to use Dazzler as a new team member, to have Lady Gaga acting along side Lucas Till, Caleb, Nicholas and James wouldnt feel right.
it would look like Gaga and the x-men, and Im sure most of xmen fans wouldnt want that feeling for this franchise