I understand that discussions in comment sections are a major problem, and more so on History:SE than on, say, Stackoverflow. I understand that this needs to be tackled because it clutters up the site with irrelevant stuff and makes it hard to identify those comments that are helpful. I understand that this causes a major amount of work and that the moderators have a very difficult job dealing with this all the time. I understand that it has therefore become standard practice to move the entire comment section of a post to chat as soon as a discussion develops, discussion being understood as contributors replying to each others comments.
However, I feel that comments are sometimes moved to chat so fast that I find myself considering whether to add comments at all. In particular for comments that require some effort, comments with additional information or suggestions how to improve the question or answer. I do not think the chats the comments are moved to are normally read by anyone, least of all the author of the post the chat is about. Further, they are automatically deleted if they have less than 15 comments and there is no new activity for seven days. (There rarely is.) If I can expect my comment to be buried in such a chat (or deleted directly, as it happens) within a few hours, it does not seem worth looking up sources etc. This also defeats the purpose of comments.
To be clear, I do not want to dispute that comments are barn cats. It is clear that they are meant to be temporary. Nevertheless they have a purpose and we should make sure that they can fulfill this purpose. In terms of T.E.D.'s metaphor, if our barn cats die so quickly that they are ineffective at pest control, this might be a problem.
Why I feel this is bad
It encourages posting comments as answers instead. My feeling is that we are seeing an awful lot of very bad answers: Short comments, quotes copied uncritically from somewhere, or brief explanations without any sources or references. Sometimes this is even the case for long-time contributors with a lot of reputation.
It is exploitable. If I would, for whatever reason, want to post a ludicrous answer (say, to promote a particular ideology), I can expect to be called out on this and the answer to be downvoted. However, if comments flagging my answer as biased are moved to chat as soon as some discussion arises in the comment section (because then all comments are moved to chat), I can simply start a discussion and wait it out. Within 3 weeks, there will be no trace left of the comment I wanted to get rid of, my post will perhaps still be the only answer to the question and people will start upvoting me. Easy. Nice way to promote antisemitism for instance.
It discourages putting any time and effort into comments. For instance, if we have a new contributor posting a bad question, wouldn't it be nicer if she got some personalized feedback from someone who takes some time to do that. Instead, she can expect the standard comment, copied for the 50000th time. Another example: If I comment about some aspect missing in an answer, should I just mention this aspect or should I look up and suggest a reference? If the comment will be gone within a few hours, it does not seem worth the effort.
I realize that the author of the post should integrate any relevant information from comments into the post. You could argue that comments should therefore be removed as soon as the original author has seen them; if she has replied, it is clear that she has. And if she deemed it relevant, she would have integrated the information in the original post. However, this is very rarely done. I suspect this is because 1. It takes effort and time (that you may not have every day); 2. It feels wrong to many people; like appropriating other people's contributions; 3. It is not usual to see other people do that.
I also realize that you can edit posts directly to integrate additional information. However, I feel this option should only be used if you are really, really sure that what you are adding is uncontroversial and does not change the spirit of the original version of the question or answer.
Note that there was another recent discussion about this. Maybe this shows that other contributors (in that case Gort the Robot) are concerned as well (although the issue was deletion, not moving to chat).
I understand that there are other opinions on the roles of comments and answers: A comment (how ironic!) by LangLangC here suggested that comments with "half-answers" should be answers, not comments. I respect this opinion, but I fear what that would do to the quality of the answers on History:SE and therefore I believe instead that some answers that only contain "half-answers" should have been comments instead.
There was in fact an entire discussion about whether "answer-comments" should be subject to arbitrary deletion like other comments. Some opinions were collected, a comment linked similar discussions on other SE sites, but there did not seem to be a consensus. Curiously, no one mentioned what I would think would be the obvious way to deal with "answer-comments" with "half-answers" (integrate the relevant information from these in the post they comment on, then flag them for deletion).