Is this a possibly abuse because they want to keep their precious rep points?
No offense, but that is laughable. NO.
As you pointed out, it is high rep users doing it more. I, speaking for my self as a high rep user, do not care about the precious rep points. So I can assure you we are not writing a so-so answer as a comment to protect our selves from DVs.
There have been many times I answered in a comment, just because I did not feel like writing a whole answer.
As iKlsR pointed out, this is a Q&A site. In this context, at the end of the day does it make a difference if the user gets his answer from a comment or a real answer?
As cegaton stated, a lot of question are very low quality - as in "I have no idea what you are really asking, but here try this."
If I had to guess I would say that this is what you are looking at when you say there are "answers in the comments that might not be very complete."
Well if I'm not sure what the question is, then no I'm probably not going to take the time to write a full answer, when the OP will just come back and say "thanks, but no that is not it what I mean is..." (has happened to me before, and that kind of thing does not lend its self to be repeated.)
In my mind, there are two main reasons to "answer" in the comments.
- The question is unclear, and any answer is just a guess.
- I do not want to write a full answer, especially when one line will do.
Now as to whether or not you should flag them. Lets examine the comment flag reasons.
rude or offensive
Flag any comment that is something a normal and reasonable person would not post.
If it adds nothing to the post flag as this.
Common example on BSE is a comment asking for the blend, where the post has been edited to include the blend.
often used on comments like "Thanks" or one of Iklsr's memes.
any time there is some problem that a mod needs to act on. Examples include a whole string of comments that should be deleted, or maybe an argument between users.
There is no comment flag reason that states "This is a guess answer. I feel it should be a real answer so it can get DVed."
Thus no, there is no reason to flag an "answer" in the comment. Either it helps get to the answer, or it will prod the OP to make it clearer.