I actually prefer the big Marvel properties (Avengers, Spidey, X-Men) being separate because too many crossovers would just complicate things, especially for non-fans. Also, though Marvel maintained continuity across all books, it always required suspension of disbelief when a huge problem occurred and you wonder where say, the Avengers or The Fantastic Four were to come lend a hand. You can't have too many characters intersecting in the movie world because some things are more plausible in print than on the big screen.
X-Men already encompasses a huge enough world to supply many movies, though besides Wolverine they don't have the solo stars that mainstream audiences would recognize. Also, their already-flimsy "mankind fears mutants" motivation would become even flimsier if all these other heroes were beloved but mutants were somehow seen as a menace.
Spidey was never really an international hero, he just stuck around NYC. Though he does fight people with projectiles, his fighting style doesn't fit in aesthetically with villains from the other comics. Basically, he's not super-powerful like some of the other Marvel characters. He always fit in better with characters like Daredevil or the Black Widow whose abilities were limited. His main mode of travel, web-slinging, isn't even possible in any city without a healthy amount of skyscrapers. Spidey in LA? Time to rent a car!