Hi Woody.
Well, first of all, you're correct. It's not for everyone and it's no big deal if you don't use it, as long as you are happy with what you're getting.
Also, it's annoying to have to explain the AF operation if you hand the camera to another to take a pic of yourself, for example.
But- and it's a biggie, for movers it makes a huge difference. Also, if shooting a fixed subject but when there's foreground or background that's distracting the autofocus, it's nice to leave the subject in focus without worrying about what the AF will grab when you shoot.
I had to be talked into trying AF-ON, but boy, am I glad I adopted it. I guess it doesn't make much difference a lot of the time, but if you shoot moving targets, or anything really that's taxing the AF system, it's something that's worth looking at.
I sometimes chip in on topics like this because in my own case I wish I'd found out about it a few years earlier!