The X-men are no strangers to welcoming new members into the fold. The rosters of their various teams have swelled over the years, even after the events of House of M where the majority of mutant-kind lost their powers. Its sometimes had to keep track of who's on X-men Blue, who no longer takes cases for X-Factor, whether Fantomex is a member of X-Force again or if X-treme X-men was a cool name, or a really corny name.
Another aspect synonymous with the X-men is unrelenting misery in the form of unfortunate events that befall our heroes. Historically, a lot of people want mutants dead and if you were to sit and read all of the X-men story arcs since House of M in one sitting, you'd be very sad by the end of it.
So when you need help and most of the planet hates and fears you, who do you call? The Avengers? Perhaps. The Defenders? Maybe. Power Pack? Erm... What you actually do - is pad out your roster with a few villains. Let's talk about these strange bedfellows.