This is interesting. I wondering something similar recently. I think that during the very early stages, it is easy to justify a vote for someone hopeless on the grounds that "I really liked him in XXXX" or "Because he's fit innit" because there's nearly always someone else that's at least almost as bad. (By the way, just for the hard of thinking, "XXXX" isn't actually a programme.)
As the competition progresses, we tend to justify our continued support with, "Well I'd rather see him/her dance again next week than x, y and z even if they are technically superior." Which is, of course, fair enough. I may even have done this myself once or twice...
However, once the series gets closer to the final, I'd hazard that most people start changing their approaches to voting. The old "I do still like them but they're really not good enough to win" mentality asserts itself. Most of the voters, while they're happy enough to keep inferior dancers in to ensure they get to watch them for another week, this becomes less relevant as the weeks become fewer.
This is why we've never had an outrageous result. The best dancer often doesn't win, but it's never been at the expense of someone who was vastly inferior. The closest to that happened in series three and Darren Gough had progressed so far from what he was that even I didn't begrudge him the win. And I have a little shrine in my bedroom dedicated to Colin Jackson. It has a little voodoo doll of DG but I can't bring myself to stick pins in it...