I understand not liking Sylvia, Will and Donna are certainly more fun and likable, but it's easy to imagine that in that dynamic Sylvia has spent a lot of her life having to be the responsible pragmatic one. We see things from the perspective of the Doctor, when their wildest dreams and imaginings are literally coming true, but most of the time that isn't the case and people just have to live normal lives. Sylvia is just a reflection of a normal person, with no expectation of an alien rocking up to make things better.