It highly depends on the question and if I'm interested in it or feel strongly about what they're asking. Sometimes I answer just the poll so the asker has extra statistics but I have nothing else to contribute so I don't leave an opinion. Other times I will do both but I never post just an opinion and not do the poll. A lot of polls speak for themselves and you don't really need to say more than what you chose in the poll.
It depends. I mostly just wanna vote on the poll, but surprisingly many people end up asking two different questions on the same poll so there's no way you can actually answer it. Other times the poll options are just too biased or retarded, so in such cases I sometimes just leave an opinion without casting a vote or more probably just ignore the question altogether...
It'll depend upon the quality of the content. You won't get any xper point for voting so most likely people would like to opine on questions but the would vote too as a mere click doesn't take much time and sometimes there are great results hidden in the poll.
So ultimately, it would depend how alluring your question is. :)