We currently do not have an official guideline on how answers in comments should be handled. And as someone who is (or was for quite some time) quite active on WorldBuilding, RPG and other sites (for example Writing, IPS, SFF, ...) I remember lots of discussions about this all the time.
Basically it often boils down to roughly the following points:
- Answers in comments can't be properly voted on -> some people "fear" the downvotes on normal posts and try to avoid them by writing in comments; this kind of behaviour is not okay
- Answers in comments are hard to read, because it's this little bit of space under the question that can get crowded if many people post comments; especially when you write multiple comments and some get more upvotes than others they can get out of context once there are lots of comments and some get hidden
- For a normal user there is no way to edit comments after a few minutes
- People often use this for not-really-researched "answers" that are more opinions than actual answers based on facts and research, which is not useful and not what comments are for
- The idea behind comments is to ask for clarification and encourage the writer of the post (question or answer) to improve their post; answers in comments don't encourage such behaviour and are therefore not what comments are made for; the answer box is made for answers and should be used for answers; the comments should be used for comments
- Comments are meant to be removed all the time, which means that it won't be bad for your account if lots of comments are deleted as long as there are no rude/spam flags on them; removing lots of answers because they were downvoted can lead to answer bans; anything with the goal of circumventing bans of any kind is bad
- You can simply remove your comment, which makes comment-chains hard to understand (answers in comments often lead to some sort of discussion about the "answer" as would be normal under a real answer)
- Flagging is different for comments, for example moderators can't add any information when they decline a comment flag and marking a comment flag as helpful will automatically delete the comment, which means that the feedback you can get from borderline comments is more limited than what you could get for borderline answers; this makes it difficult for everyone to understand decisions relating to such instances
- Comments are temporary; it would be fatal if someone asked a question, someone answered in the comments, everyone thanks them for the "answer", the comment disappears and later someone has the same question - and can't find the answer anymore...
- You can comment on closed questions and thereby "answer" questions that the community deemed to not fit the sites scope
Much information can be found here: How do comments work?
The gist of it:
Comments are temporary "Post-It" notes left on a question or answer. You should not expect them to be around forever.
I think we should not allow answers in comments.
But before nuking the comment-answer you should try to ask the user to expand their comment to an actual answer, because comments are normally too short for a real answer, so that their information will be preserved for future readers with a similar question.
RPG is kind of extreme about comments. I don't think we need to be that extreme here. But we shouldn't let everyone do what they want to do. The existing features are there for a reason. If you feel there is a problem with normal answers that makes you use comments instead raise that issue on Meta instead of willfully using the wrong feature to get around some restrictions.
But always remember that not everyone is used to StackExchange rules. Explain the guidelines, tell them the reasons and if they do this time and again raise custom flags and explain the behaviour to mods.