Imagine a drunken fratboy locked in a closet, his first encounter with a girl. He desperatly, and shakily searches, trying to figure out what to do, dizzy and bewildered things just get worse for him, his beer, the girl, and the closet.
That's pretty much how Claremont has handled any additions to the X-Men continuity that don't stem from himself, Lee or Kirby.
Magneto/Xorn was just some imposter (which, overall does help with the fact that 'Magneto' was having trouble with his powers at the time). There's a real Xorn that the adjectiveless X-Men have just encountered in China who's brain has turned into a black hole (coinciding with 'Magneto/Xorn''s backstory). ImposterNeto killed Jean with a lethal EM pulse in her body. Jean is coming back in the Astonishing book soon, though. ImposterNeto threatened, although jokingly and crazily, but still a threat nonetheless, that his secondary mutation was to always come back, so we may see more of him. There really needs to be some sort of reckoning between Excalicrapibur, Astonishing and Adjectiveless X-Men at some point soon. That would be a good idea actually, every quarter put out a themed book, all the X-teams get together for a giant meeting, to see whats going on, and of course, just to give us our money's worth, a giant fight or something. Same with Marvel Heroes, and if you really want to stretch it and rape our pockets, Knights too.