Way too simplistic, I suppose, for this discussion, but I sort of follow the jazz theory that whatever sounds good IS good. I'm not skilled at harmony, but it comes in very handy when others are singing in a key I am uncomfortable in. And God knows, it sure feels good when it's right on the money.   But there are times, to my ear, anyway, when coming togther in unison, especially in tight phrasing, is exactly the right thing to do, just as sometimes, in some songs, dropping the harmony altogether, in certain secitions, works best.

And there have been times when singing in 3rds gets sort of monotonous, to my ear....and unison is a relief.

If it sounds good, I'll welcome anyone on my note.