I believe there are legit reasons to provide an answer (or sometimes a hint) in a comment if:
- the question should not be answered (it's silly, duplicate, off-topic etc.) and either
- the question lacks basic understanding but is asked in good faith and deserves at least a hint, or
- the question asks about something dangerous and a quick warning is in order
This way, the question can still be safely deleted, but the OP gets at least some reward for their effort. And if he's about to do something dangerous, he gets warned quickly (this applies more to EE.SE, where people ask things like "I'm trying to connect this circuit to 220 Volts", and you instantly see they shouldn't, but I can imagine similar cases relevant to U&L).
Additionally, it also happened to me to see a question I didn't know the answer for, see the comments, google around and finally post a complete answer based on someone else's comment. I don't see anything wrong with this practice, and nobody ever told me that I've stolen their answer, so I don't agree that comments discourage real answers to be posted. On the contrary, if the author of the comment didn't write it, the OP wouldn't probably get a full answer at all, or at least not as quickly.