This comes down to what our priorities are.
- Do we think no answer at all is better than a poor/partial answer in the comments?
- Do we think poor/partial answers is better than having the same in comments?
Two of the usual reason that Stackexchange does not like answers in comments is that comments are ephemeral and may be deleted. But this can also be an advantage.
History.SE attracts a lot of complex, multipart questions. It's perhaps the nature of the subject that people tend to have interlocked queries, and post them as one. For example, this contains two main questions and four subsidiary ones. Often people might know about or only have time to elaborate on a specific part. If they were to share this knowledge, then we'll either get partial answers in the comments, or incomplete answers as a reply.
Similarly, it's not rare to see questions that are misinformed or confused about certain critical details. Often a quick clarification could assist the asker in identifying the real question of interest. Again, this could be a partial answer in the comments that clears up a specific part of the question, or an incomplete answer that may easily be rendered irrelvant if the asker were to edit their question to improve its quality.
Given the choices, I believe comments are superior because they don't clutter the thread with partial answers. It takes less time and effort, and doesn't come with the risk of being penalised with downvotes, therefore we encourage more participation and exchange of information. Lastly, and this is where the ephemeral nature of comments come in handy - these can be deleted without much impact.
As an addendum, I would suggest that discussion comment chains should generally be moved into chat with a corresponding link left in the comments, rather than deleted completely.
Another scenario is trivial questions, which H.SE maintains a policy of closing. It's often easy to explain why such questions should be closed by answering it, which I believe is superior to leaving the asker in the dark. These could be posted as full answers, but in the common case where the answer is a Wikipedia article, the standing consensus is that we do not want to duplicate Wikipedia, but also that link only answers are not allowed. That leaves comments.