Only rarely, but I do use -- votes for clearly incorrect nodes that I felt showed a lack of willingness to learn something I thought was important. Something that should have been obvious from what else was going on. (Such as the surrounding conversation.)

So I vote down nodes almost entirely on content.

BTW I use a similar (though far more generous) criteria for ++ votes...

EDIT
Ugh. The "similar" comment is unclear. :-(

I meant that I vote nodes up based almost entirely on content. I am particularly generous when it is something that I thought was important, or something that I learned from.