Probably some good 'ol boy doctors do it, and they'd rather have cash-in-hand than an IOU. On the flipside, I've had to write collections letters to specialty drug patients whose insurance lapsed for $60,000.
I haven't been turned down yet, though the discount varies. This is in a major West coast city, not out in the sticks.
It is a metric fuckton less paperwork and hassle for them to just get paid now.
If you are, as I guess most observers are, a "pay as you go" type of person it never hurts to ask.