Like they say, any publicity is good publicity. It definitely encourages reading. It lets the general public out there aware of the book the movie was based on. Even if the movie was bad, anyone with an open mind wouldn't let it affect their impression of the book. For example I liked the idea of the movie John Carter so I read the books after watching the movie everyone agreed was bad but I fell in love with the series.

PS: Three Musketeers is a FANTASTIC book but the movie is an absolutely horrible adaption