In Days from Future Past, we saw how Kitty sent Wolverine back years earlier in time (1973) to prevent Mystique from killing Trask and thus stop the Sentinel program before it started.
Assuming that Kitty isn't the only one with the time-traveling gift, this could create chaos in the mutant world. Why didn't they change anything else?
For instance, Xavier could have sent Wolverine earlier in time and prevented the fight between himself and Magneto when they were young. This way, he could have prevented himself from being disabled and avoided a lot of pain. Also, Wolverine could have killed Stryker in that earlier time and avoided a lot of pain for himself.
And what about those myriad other mutants who are not related to X-men. What if one of those travels back in time and kills Xavier in his childhood? This could result in chaos in the X-men universe with multiple unexplainable and unrelated timelines.