Some of the boyband members that have sold lots of records, appeared at Award shows and had headlining gigs/concerts have generally done okay, but some haven't. Harry (Mcfly) and Jay (The Wanted) won, but Simon (Blue) was in the bottom two a couple of times (needing judges help), while Nicky (Westlife) didn't really get near the final (despite his band selling loads in the UK). So, sometimes the fanbases help, but I'm guessing Harry and Jay also won lots of new fans from Strictly, and so I think the show's overall fan numbers are really important in balancing out the uber fan vote.
I think, given that the best technical dancer hasn't always won, personality/popularity are determined from day one of the show and celebs with sizeable prior fanbases don't always figure so massivley, as the celeb is not doing what they were famous for, they're doing something for fans of the show to newly become fans of/or not. Even old fans may not take to their dancing (over another) despite liking them alot (for what they have already proved to do well).
Strictly does have a range of fans, age wise, so some celebs, like Daniel O'Donnell won't be known by all of the show's fans, so it's only the dancing some voters can go on (if have not prior loyalty) and if celeb aren't good at dancing or at least engaging to some degree then no previous music/acting etc success is going to matter.
If the celeb is not engaging as a dancer/person then even a popular Pro can go out early and can't save them. Pros' seasons depend a lot on their partner, as one year they're a winner and the next year they go out midway or before. Only Kev really seems to buck that trend, though he's never had an awful dancing celeb.