After a second viewing, I think I finally figured out why they blew up the hospital. It was necessary in order to provide the "kick" to wake Ariadne up from the fourth dream/limbo. (It was supposed to wake Cobb up too, but he chose to stay there.)
My interpretation of the last scene of the movie is that Cobb decides he no longer cares if the top will keep spinning or not. He chooses to see his children's faces again-- something he had been distracted from and afraid to do before. This is the reality he has chosen, where he wants to be, and whether the top spins or falls is meaningless to him now. The final shot is just meant to get the audience thinking.