Public versus private though. If my adviser came in with a printout of something I wrote and told me, "This is garbage and unworthy of the electrons used to send it to me," I would be OK with that -- but if he stood up at a conference, put it on a projector, and said, "My student wrote this worthless crap" I would be (rightly) angry about it. Insulting someone's work to their face in a private setting is not nearly as bad as publicly demeaning their work.