Word of mouth will make or break a film, but it works both ways. It can give positive feedback to a small film that really deserves it or it can crush an over inflated piece of crap that really doesn't warrant it. Films cost an absolute fortune to make, these days, it 's the advertising $£'s that will make that money back for them.
Of course a stupid amount of money is spent on advertising these days, for anything. A lot of entertainment is competing with a lot of entertainment. Film doesn't have to worry about TV any more it has to compete with gaming. Call of Duty (very heavily advertised) is the biggest entertainment IP in the world right now. Sure, games cost more, but films come and go in a matter of weeks and are replaced with the next money spinner. People don't go nuts for films like they used to, after Harry Potters last movie, will anything really get people that excited again?
There's a fu'ck tonne spent on advertising these days, out of pure necessity.