Prompted by a chat discussion.
Sometimes, users post answers in the comment section. They do this for several reasons - some common ones include that they don't have time to write a full answer, that they are only addressing part of the question, that the answer they wanted to post doesn't have sources so they're just leaving it as a comment...
Should History.SE be deleting such answers as comments?
Personally, I think that answers posted as comments should be treated as any other comment. That is they're open to deletion as and when the 'system' considers it necessary. I can't see any reason for giving them special treatment.
As noted in the question, these are often partial answers that, for a range of reasons, are not strong enough to stand as full answers on their own. However, they can have value in providing additional information that can guide the questioner or others to a complete answer. Alternatively, they can be links to complete answers elsewhere (such as Wikipedia) and we have a policy of not allowing link-only answers nor reproducing those answers on this site.
Depends on context. If I purge a comment stream because it is chatty or acrimonious, then "answer" comments are going to get purged too.
I've probably submitted more "answer-comments" than average. I don't like to answer a question unless I can provide a researched citation. I accept that my answer-comments are subject to arbitrary deletion.
Comments are ephemeral.
This comes down to what our priorities are.
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.
