Its fine to post comments if user cannot answer in detail, but if he is confident enough to answer the question then he should answer instead of commenting
You yourself answered your own question "answer in short" so we don't want our users to answer in short (comment like) and hence they end up posting a comment.
Now is this good? No, posting answers as comments is not a good thing, but if a user is not confident enough to post as an answer, it is completely fine to post as a comment, it will somewhat help the user to refer something rather than the user posting a wrong answer which further affects the quality of the site.
Users generally comment or should comment when :-
- They need any clarifications from the user who questioned
- They are not sure about the whole question but still want to help a user with a part of his question
- They don't know the answer but still want to add something to the post
- Falsify any claims / information provided by a user in his question
- Cannot answer in detail (important)
Users should post the comments if they cannot answer in detail, just like I mentioned in my previous post, so even if user tries to answer in 2-3 lines and it is not much useful, his answer will be down voted (probably) and will be deleted eventually.
So if users should answers if they are sure enough to explain the OP in detail, if not, it is recommended to leave an helpful comment if he knows some part of the question.