I am coming to this question from this one by Remi.b on essentially the same topic. In the interest of maintaining the SE spirit, I've voted to close the other and answer here, as I must have missed this question the first time around.
I've been a member of Biology for over 4 years, with various levels of involvement over that time, and I've pretty consistently been a very involved user of Stack Overflow for nearly as long, as well as a number of other SE sites with somewhat lower rep. So, I've seen a lot, and tried to stay on top of "acceptable practices" and whatnot. On pretty much any other site, "comment-answer" answers would be pretty quickly downvoted and deleted, or massively edited to remove the comments.
It is my view, and (I think) the view of the overall SE community, that answers are for actual answers to the question being asked. This is not a forum, where each next post in a thread (they're not even labeled questions and answers) is the continuation of a conversation, and you have to dig to find the actual answer(s) to the question. From the very beginning, SE (well, SO, at first) was designed to be different. From the tour on every site, at the top:
"This site is all about getting answers. It's not a discussion forum. There's no chit-chat.
I think making long comments into answers (I'll admit, I've done it once or twice) directly goes against the network's philosophy. It's a little fuzzier when an answer is included at the end of the long comments, but I feel that comments should be restricted to the comments section. On other sites with a lot more traffic (SO for example), this is mainly to prevent very low-rep users that don't yet have the 50 rep needed to comment on any post from commenting in an answer. These get downvoted and deleted fairly rapidly, most often. Comment-answers like we're talking about here, mainly from high- to very high-rep users, are quite rare, as high-rep answerers generally know the rules.
"But I'd have to write many comments to get my point across!"
Brevity is the soul of wit (from Shakespeare). Instead of writing long, descriptive sentences, be terse. Make bullet points, not drawn-out arguments. Use multiple comments, if needed. When I'm doing that, I leave enough room to put
(...) at the end of a continued comment, and at the beginning of a continuing comment. There is absolutely nothing wrong with writing multiple comments; if you have multiple issues you're addressing, you may need to. You also need to very thoughtfully consider your comments before posting, not only so you can catch mis-spellings, grammar mistakes, etc., but so that you can be as succinct as possible.
"But I'll lose my nice formatting!"
Too bad, so sad. Learn Markdown and judiciously use italics, bolding, and inline code/monospace text to highlight things.
"The comments may be out of order/someone may post in the middle of my comment series!"
The first is handled simply by going to the bottom of the comments shown and clicking on the
Show X more comments link. If you don't think the OP (original poster) is intelligent enough to do that, do you really think they'll thoughtfully read your comments and take them to heart? The second part is handled by writing out all your comments ahead of time, preferably in a text editor that provides Markdown highlighting and character counting, then posting them one after another. You may trip the captcha, but once you satisfy it you won't be bothered.
"What about making my comment-answer a community wiki?"
That addresses the problem of gaining/losing rep based on your comments (no rep is gained or lost when true comments are voted upon - you'll also notice that you can't vote down a comment), but you're still posting a non-answer as an answer, which is clearly against the philosophy of the network.
"But sometimes I post some general information answering the OP's question(s), or point them in the direction of learning more!"
Two things: first, from How do I write a good answer? in the help center,
Answer well-asked questions. Not all questions can or should be answered here. If the question is bad, or shows little or no prior research effort, or whatever, don't answer it. Doing so merely encourages the same person and others to post low-quality questions, knowing that they'll get answered anyway. Feel free to point them to another resource that answers their question (if possible), but you should not be answering it here.
Second: If you feel the question is of high-enough quality to answer, but it has formatting problems or whatever, feel free to edit the question into shape, and then answer it (assuming it meets all criteria of being on-topic, showing previous research, etc.), and avoid editorializing in your answer. Comments are the place for that.