Lets face it, they can only do so many Batman movies before they start repeating things from old movies, and we don't want that (Batman Forever/Batman and Robin) So why not make a HUGE move and join forces with Fox to do one huge franchise making film.
Reason #1:Opening up for new exciting stories
Wouldn't it be just amazing is Superman and Cyclops faced off in an Optic Blast contest.Doing a movie with characters from two different comic book companies would open up the doors for not only team fights, but team-ups. Imagine Lex Luthor and Magneto taking over the planet. Plus, FOX owns Fantastic Four which opens up for a huge (Don't hate on the name) Fantastic X-League film.
Reason #2: Not that I want this, but they could take Marvel down
A huge franchise team-up like this would give the Avengers a run for their money. Who knows, maybe SONY would get in on it and let Spidey join the team. It could also allow MARVEL to realize what they did by selling their characters, and in the farthest reach, could open the franchises to one big smash-up film with Avengers, Justice League, X-MEN, Fantastic Four, Spider-Man, and anyone else who wants in on the fun (Hellboy, you know you want it)
Reason #3: MONEY!
Pretty self-explanatory. Huge movie done right means huge money. It's not Algebra, which is hard.