It's the real third party. People just want to be on the winning side so they'll cynically vote for the side that they think is going to win. You see it every election when you start to get some momentum to either side. Or in sports. Everyone just jumps on the bandwagon

It's actually the more irrational thing to do since you should actually vote for the side that you think is going to lose, IMHO.

This is a good question. Polls might be used as a tool to moralize or demoralize, or convince others that a predetermined result is legitimate. Given the connections the media has with the government, I suspect the later, rather then the former in this election.

Yes, in at least two ways:

Feels good to be with the wining tea, same as sports as nb2kk says, but also in another more insidious way: if all the polls say candidate A is a shoo-in, people who might vote for candidate B are less motivated to vote at all.