I noticed many people with high rating tend to post their answers as comments. Doesn't this defeat the purpose of the site (by not allowing an answer to be accepted)?
Many times answers in comments are not complete answers but more of the form: "Have you tried X?". Which is really more of a comment than an exact answer. From my perspective this is more common for vague questions, where you might have several "Have you tried X" type comments and don't want to clutter up the question with several answers that no longer apply.
If I have an answer, I'll answer. If the question doesn't contain enough information to answer conclusively, but I have a "try this" suggestion, I'll probably comment.
This seems to be the case more often than not - comments are more like Q&A with the OP, and the question may end up being answered that way. But clearly written questions usually get answers, not comments.
Sometimes if I don't have time to answer properly but have the answer, I'll give the answer (or at list a pointer) as a comment. Ever since I went over 10k rep, I don't care about points that much.
The other case where I would answer with a comment is when I'm not totally sure about my answer, either because the question is unclear or because my knowledge is a bit off on the subject.
Replies by posters with higher ratings get more attention and even more rating, thus discriminating (to certain extent) perfect posts of people with low score rate. Next, perfectly valid (according to rules) posts are sometimes downvoted by certain groups of people (such as open-source evangelists) who think they are above the rules. To avoid this, it's better to answer in a comment.
One explanation. Some of us, upon seeing a question of questionable validity, feel a dilemma. We might know something useful. But we don't want to encourage poor questions by answering them. And often, with low-quality questions, it's quite unclear what's really being asked, and so any answer has a good chance of being (ahem) pointless. There's also a certain hypocrisy about answering and also voting to close. In these cases, a comment is a way of offering some possible assistance to the OP.