Answers should be quality. Things like references, proper formatting, etc. should be included to aim for quality (goes for Questions too IMO).
Sometimes I'm not inclined to leave a quality answer, so I just leave (what has turned out to be) an answer in the comments alone. I helped, the user likes it.
If someone else wants to come along and write a detailed answer with nice links, formatting and spell-check, etc., based on the comments (or just the same knowledge) then more power to them -- and I hope they go edit 50 more questions or answers to improve on them in the same ways.
Just don't be a jerky and pirate the answer from the comments 2 seconds after they show up. :)
On the flip side -- I don't think I've ever actually done it to someone else's comments (maybe?). When I see someone has left the answer in a comment I'll usually add a comment @ them, saying "you should make that into an answer".
In the end, I believe the goal of the Stack Exchange sites is quality Questions and Answers, not Comments. So an Answer is better than a Comment - regardless of the author.