The primary harm of discussions is that they distract from answers. If there’s discussion in addition to answers, that’s ok — and that’s why i advocate a hybrid system between SE’s comments and chat. The problem is when the answer gets buried in discussion. Surely you must have seen these forum threads that go on for hundreds of pages, and there’s a plausible answer on page 3 but in fact it became obsolete a couple of years ago and the current answer is on page 176, but good luck finding it. Q&A sites are much better than discussion forums at being a repository of knowledge because there are no answers to answers: replies to answers are second class, only replies to the original question get prominence.
If a topic calls for back-and-forth discussion rather than a straight answer, it isn’t suitable for the Q&A format and needs to be closed.