Some times, as I'm flagging non-answers (which I don't do aggressively, but when I see something that really isn't right, I do), I'm torn between using "Not an answer" and "Other" because "not an answer" doesn't leave me any explanation area to describe why the "answer" doesn't look like an answer, but the reasons for "other" are more vague, and less quantifiable.
In many cases, it boils down to "should have been a comment", which I know brand new users may not be able to do, which is why some "answer" instead.
And this isn't so much about being concerned that my flags will be disputed, as that's only really happened once, and that on SuperUser. But it did, kind of get me thinking, it might be useful to be able to flag "not an answer" and explain (shortly, because I know Moderator time is precious) "Question is asking about X, and Y. Answer talks about Z, which isn't really related."