Comments can arise in any number of ways, and I feel that any attempt to formalize what a comment "should be" would be a needless restriction on the unbounded creative potential of the community.
Of course there are comments that don't contribute to the subject matter and that won't be missed, and removing them improves the overall quality of the post and the question as a whole. But that should really be the guiding principle: How can the content on the page be improved, and how can visitors (as well as the productive community) be served better?
I can't speak for other languages, but in a language as complex as C++, hardly anyone can claim to know everything. Many an answer has been written in the best faith, but was significantly improved upon later thanks to a comment. I might claim in earnest that a great part of my understanding of C++ originated in comments.
But the author of the post may not always stick around or be interested in "maintaining" an answer. And why should she? We certainly don't want to load any permanent commitment on any intelligent would-be poster who chances upon our site. If an answer is valuable by itself, then it should be allowed to stand, without the demand for anyone to "continue working" on it -- we're a question-answer site, not an encyclopaedia. But when valuable information is added in a comment, that too should be allowed to stand, no matter the volume of the comments.
[As @Sehe suggests, it is an option to have some other member of the community perform editorial work on a post, perhaps incorporating comments into the main post. However, this requires someone who is both competent at the subject, diligent enough to actually improve the post without harming or distorting it, and who would actually spend the time on going back to older posts to see if they need working on — in practice this seems to happen very rarely, not surprisingly.]
The visual style of Stack Overflow already does an excellent job at keeping the visual noise coming from comments at a minimum. So if an answer is complete and good, any visitor is entirely welcome to read only the answer itself and move on. If she wants to know more and see other people's take on the matter, the comments are available on demand.
In short, I do not see the necessity for policing all but the crudest and most egregiously unproductive comments. Otherwise, if a comment doesn't mean anything to you, that's fine - just walk on by, and maybe it'll be useful for someone else.