First, they only tell you about what's going on at a given moment in time. As we've seen, time gives people more chances to stumble, so the farther out in time you are, the more likely they are to be different from the 'poll' of actual voting.
Second, they often tell you things you (and the candidates) don't want to hear, but in so little detail that they don't really give an idea of what the problems are that need to be reversed to change the polls. I keep hearing the lament 'Why is O'Malley doing so poorly'? Nobody really knows. We can all give anecdotal guesses, but no one ever does polls in enough depth to answer such questions. And without knowing, he's stuck trying to simply guess at what he should do to try and improve his poll numbers.
So the advantage is to people who don't need polls. Who base their campaigns simply on being themselves and telling their history and the future they want to create, rather than trying to create their own history on the fly, chasing polls.