What always puzzles me is the way it always gets assumed that being ahead in the polls helps a candidate.

I'd have thought it'd be just as likely to work the other way - you'd have people say "Well, if he's going to win anyway, I won't bother to vote for him, I'm not all that keen on him anyway" and even, of the trailing candidate "Well, no harm in voting for him, even if I'm not too sure about him - stop the other fella getting too smug if his majority isn't too big."