There's nothing rude about doing something for reputation except cheating, which is probably not all that worthwhile since you can't redeem those points for anything else anyway. The whole point of reputation is to indicate and enable people who are doing positive things for the community, which is where the votes come from.
I'm very guilty of posting what amounts to answers in comments; usually I do this because I do not want to take the time to flesh it out in the way I would be prone to do in an answer (I'm also very guilty of being very long-winded, so sometimes this is perhaps for the best).
Sometimes "not fleshing it out" means the comment is not of a form that I think will actually be digestible by the OP (= "original poster", generally referring to whoever asked the question). In those cases, part of my intention is to leave a clue for someone in a position to mediate by turning that clue into something more digestable in answer form.
Point being, if you ever see a comment by someone else that amounts to an answer and you have an understanding of the context, please feel free to turn it into an answer. Even more so if it is a comment by you, but do not feel that it's an absolute requirement.