In the case of DUers, I would say that they're trying to show that they're so much smarter than we are. That's what they believe.
Though I agree with pcottraux's analysis wholeheartedly, I think you're dead on as well. No matter how many "DUers are stupid" jokes we might make over here, I'd say there's not a single conservative who believes that all (or even most) liberals are lacking in intellectual capacity. However, in my experience the vast majority of libs think that conservatives are conservatives because we're dumb or uneducated. Being a troll is the attempt by some of them to let a little illumination into our benighted lives.