I noticed that although when I directly click "comment" under someone else's question I get reminded that my reputation is not high enough yet, if I answer a very short answer it gets converted as a comment on the original post.
I have no opinion whereas it's a good or a bad thing, I just wanted to point it out! If it's voluntary, why not let the users under 50 rep comment the OP's post (if they can do it anyway by posting a short answer)?