I also dispute this- obvious cases include partial disagreement and partial agreement between parties, somebody who is simply silent or who says nothing of substance, and someone who is themself trying to learn from you/the other side.

(In particular, consider a debate between a biologist and the Pope on evolution. I would expect the Pope to be neither offensive nor defensive- though I'm not totally clear on the distinction here, and how a debater can be neither- but I would expect to learn much more from the biologist than the Pope.)