Comments are just for requesting clarification and arguing with people, right? ;)
Yet lately I'm seeing this more and more often (no data available; sorry to pick you specifically, Barry):
What happens is that someone FGITWs a mini-answer as a comment on the question, diluting the value of the actual answers. Sometimes no answers appear at all, and the OP's question has been answered by a comment. So then what?
It breaks the whole Q&A model by turning questions into flat forum posts or chatroom messages. Stack Exchange was invented to get away from this messy model and instead create a system of Questions and Answers. Period.
- I don't have time to write a full answer now
Okay, write it later!
- I will never have time to write a full answer
Okay, well, don't then. Let someone else do it.
- I'm not sure enough about my suggestion to write it as an answer
Then posting it at all could be damaging to the OP. Watch other answers, learn a bit, then post an answer later if you feel you're surer about your idea.
- I just want to give the OP a hint
Invite him/her/it to a chatroom.
- I don't want the rep from an answer
Give it away in bounties to answers elsewhere that you feel are outstanding.
- The question is off-topic so I don't want to post an answer and get berated
If the question is off-topic then it is off-topic. It should not be answered at all. Writing an answer in comments both enables the naughty OP and creates a spasmic mess of anti-Q&A, surely the antithesis of Stack Exchange.
As of 2019, this behaviour is extremely widespread. I often see more answers-in-comments than answers in the actual answer section. Have we all forgotten why Stack Overflow was created? Have we forgotten the very purpose of a strict, peer-reviewed Q&A format? Are we all that desperate to "help", even on what we deem to be off-topic or trivial questions, that we are willing to sacrifice everything valuable about this site that set it apart from the others?
Fortunately, other network websites have started taking this seriously. Picking just two examples off the top of my head, Interpersonal and Workplace moderators routinely remove answers-in-comments and remind people to stop posting them. Even on SO we have "avoid answering in comments" under the "add a comment" tooltip, but this goes completely ignored and flags (last time I dared attempt one, anyway) are not only rejected but also discouraged.
I'd like us to start being firmer about this on Stack Overflow. Perhaps a special flagging reason for comments? Although I'm loathe to begin removing "useful content", this is not the correct place for them. At all!