You mentioned having read more about it on other sites' meta, but I hadn't yet, so I did a bit of research on four other non-beta metas and found that there is indeed an extensive "literature".
I'd say that just about everything I read on the other sites applies to what I know of French SE. Here's a quick roundup of some of the recurring themes, if not for your reference than for that of anyone who wants some background:
The ideal is for answers to be answers, comments to be comments.
Everyone posts answers as comments sometimes. They do so because they would rather not "leave something so brief as a full-fledged answer"; they "want to do more research to substantiate their answer but do not have time"; they "feel like their answer is a valid answer but not the valid answer"; they want to give an answer in the form of an (apt) link to a resource and such answers are discouraged; the question can't be formally answered as stated; or they are about to close the question but don't want to leave the OP "empty-handed". High-rep users are among the most frequent advocates.
"A helpful comment to an on-topic question is better than nothing -- it may prove useful to the asker, as well as to others who might be willing to write answers. Not making such a comment into a proper answer, for whatever reason, is well within your rights. I would even add that any request for a proper answer more forceful than a gentle nudge would be inappropriate."
If a question has a legitimate answer in a comment, that comment will ideally be turned into an answer — people are pretty unanimous on this. The reasons include not being able to downvote comments, making answers more easily findable, avoiding an ephemeral format being deleted, generating long and messy discussion threads, discouraging people from taking the time to write a well-supported answer when the OP has already read their response and gone away satisfied, and more. There are different ways to turn a comment into an answer: ask the user to make their comment an answer; write an answer based on their comment (either significantly developing it and giving a nod to their comment, or making it a Community Wiki to avoid garnering credit and reputation); ask a mod to change the comment into an answer... Similarly, legitimate answers, even if short, should not be converted to comments.
Comments that have generated answers in the same vein may be deleted if you choose to avoid clutter and confusion; they're not intended to be a long-lived medium anyway. But if people have responded and it's responsible for some of the context of the answer, probably leave it where it is. No mention of deletion of comments that have not generated answers but been left as they were.
I would say that just about all that's been said can apply to French SE as I've known it.
When I post a comment-answer, I do it for the same reasons: I'm just acting on a hunch or making a comment meant to suggest ways to think about the topic, not offer an answer worthy of French SE. If someone sees more value in such comments than I do, I usually write an answer eventually if I have time. But I've never yet had it suggested to me that I delete a comment because it didn't turn into an answer.
Some of the threads referred to:
English Language Learning: 1, 2, 3, 4, 5
English Language & Usage: 1, 2, 3, 4, 5, 6, 7, 8
Stack Overflow: 1, 2