I have seen this feature in some discussion board software (usually called 'quick reply' or something similar), however, it is only available once you are inside a single thread, and those boards traditionally are more linear in their discussions, relying on quoting for threading.
It would only therefore really make sense to me to use it in Moodle forums where student's can't make their own threads (assuming you mean responding to your forum introduction, rather than a post you make starting a thread), and are not in the main intended to reply to each other's comments i.e. Forums used as some kind of Blog-variant. So Martin's suggestion seems appropriate.