It doesn't set the stage or was even intended to. It was just a children book he wrote and later he decided to make much more serious books about it afaik. And the universe does change a lot. Not so much in plain fact, but in tone. But there isn't really that much in common in the first place. You've got the ring, about which they later find out isn't a handy little thing but the doomingly dooming doomthing of doom. And then there's Gandalf (I think in the Hobbit Gandalf hides on a tree from wolves for like days, in LOTR wolves hide on a tree from Gandalf). The few others are really minor characters.
I am pretty sure the movie will follow the tone of LOTR and be full of action and drama and people screaming "NOOOOOOOOOOOOOOOOOOOOOOO" and that's good but for the same reason I'd think that people picking up the book after watching it will be like wtf.
It does set the stage, it's the beginning, just because it was not originally intended to do so does not mean that it now isn't. He adapted the original The Hobbit book to accompany the future books after he was approached to write more. Any fantasy universe made this way has a beginning where it does not detail everything, a single book, just one story, which is the beginning, i.e The Hobbit. The universe did not "change alot", because there was not much of a universe yet, just that alot more of the universe needed to be created, it was and it flourished.
Yes, things like characters and/or actions differ in The Hobbit because it is tamer and was originally intended as a kids story, but it still doesn't mean it is any less a part of the tolkien universe "middle earth", just an immature unsure start. But the basic route of the story will be there, even if the movie will probably add and alter a bunch of stuff, it's still going to be "The Hobbit", just one more fitting to the current Universe and also the "state" of the movies, i.e more fighting, a more entertaing movie for adults aswell as kids. The LOTR movies have alot of shit that didn't happen in the books and also miss out alot of stuff that happened in the books, but they are still awesome movies.
