They were wrong in 2000 and 2004.
Polls can reveal underlying or emerging trends and help campaigns decide where to focus. The danger is that commentators use them to declare a race over before the votes are in. This can demoralize the underdog's supporters, depressing turnout.
In truth, however, no one knows for sure what kind of polling deficit is insurmountable or even which poll is correct. All of us should act with the proper understanding that nothing is yet decided.