I actually prefer having X-Men and Spider-Man with different studios. This way you get the studios all trying to one-up each other, and you get more movies and more universes built up rather than if they were all just under one roof. It would be nice seeing Spider-Man with the Avengers, but the Avengers are plenty stacked as it is. If it were all under the Marvel Studios umbrella, we wouldn't be seeing an Age of Apocalypse movie alongside a Thanos saga alongside a Sinister Six build-up like we are now.
Also, honestly, I've found the Marvel Studios movies to be getting awfully repetitive and formulaic. They're all solid movies, but I go into them feeling like I've seen them before. I've been looking forward to Guardians of the Galaxy for that reason because I feel like it's them actually trying something different.