It actually happens quite a bit in movie musicals where the actor/actress playing the role doesn't do the singing (and even though it's a little disappointing that they don't sing their own parts, it doesn't always turn out bad). Minnie Driver didn't sing her own part in Phantom of the Opera. Most of Audrey Hepburn's singing voice was someone else singing in My Fair Lady. Even in The Sound of Music, a lot of Christopher Plummer's singing voice was dubbed by someone else when he sang Edelweiss at the end of the movie.
Movie studios have chosen star power (or maybe good acting) over good singing in the past. Even when the role required singing throughout most of the movie. Why did they cast Gerard Butler as the phantom, one of the most iconic roles in musical history, even though he had zero singing experience before the movie? And, I have no idea why they ever thought it was a good idea to cast Pierce Brosnan in a singing role for Mamma Mia (now that is one person who should have been dubbed!).
Don't get me wrong, I'm not saying Emma Watson is going to be a bad singer. In fact, I have high hopes for her. I'm sure she will surprise us all. I'm definitely not going to judge her before I even hear her. However, knowing how some movie musicals have been done in the past, it's understandable why people would be worried about who is playing/singing in the main parts, and how they are going to handle the singing, especially when they choose a star who we never knew could sing before.
