There is already a significant amount of discussion regarding answering in comments in meta, but since you put up some specific points, I think it may be useful to address them.
All "situations where this might happen" are undesirable, in my opinion.
Before explaining why, let me preface by saying that many of us (inclunding me) is guilty of the sin of answering in comments from time to time. But it is not a good practice, and we should strive to reduce it.
Now, let's see:
- Someone thinks they know the answer, but isn't fully confident so doesn't want to post an Answer that might be wrong.
While this may make some sense from the perspective of the one who is commenting, this is bad for the community. For instance, the comment (by the very rationale which you mention) might have more chance of being misleading while also avoiding proper voting that could point to future users that it is problematic. (Recall that comments don't have downvotes.)
- They have an answer, but not the time and energy to write it up in detail.
Then one should refrain from writing anything at all. Answering in comments like this makes it less encouraging for people to post satisfactory answers in fear of just being unnecessary elaborations of what was commented. For an user who does not have comfort in the subject at hand, an incomplete comment may even be harmful.
- They're not 100% sure they understood the question correctly, so the answer is provisional.
Then one should primarily ask for elaboration before answering.
- They don't want to clutter the page with possibly trivial answers.
If a question has a lot of answers which are similar to the potential answer, then answering in comments is also cluttering. If the post has no answers (or no similar answers), then answering is not cluttering. (Note that the answer being trivial or not has no a priori relationship with cluttering.)
- A wrong comment can't lose reputation points, while a wrong answer can.
This is arguably an argument for answering in comments being undesirable, since it says that one could do this in order to avoid the consequences which the system has in place when you properly follow its protocols.
- Answering the particular question requires interaction with the questioner.
Arguably, no question should require interaction with the questioner to be answered. If it does, it is because some of its context must be clarified beforehand. Then one should proceed to do so and after that answer it in a proper answer. If there are points in the answer which the questioner does not understand, then they can be elaborated upon in the comments under the answer and possibly in chat if the discussion gets extended. This also allows a more focused discussion regarding the points of an answer. If this discussion happened in the comments of the question, it could be cluttered among a lot of comments with potentially different objectives.
- They want to see what answers other users come up with first.
I don't understand the rationale behind this one. Answering does not inhibit others from posting other answers.
- They feel that their answer is relevant only to the questioner, so needn't become an Answer.
An answer is relevant to everyone who can potentially have the question at hand. If an answer would be problematic because it is relevant only to the questioner, then the actual problem most likely lies in the fact that the question is relevant only to the questioner, in which case it may probably be of low quality. But regardless, the problem lies on the question and not on the answer, and the answer should not be in a comment.