Funnily enough I've just replaced the QR seat clamp on my mountain bike with a Nukeproof non-QR one. I struggled to do it up tight enough to prevent the seat slipping. However, I'm pretty sure that the clamp was as old as the bike, which is quite old and quite high mileage. It's a Giant XTC 4 which I bought used and I think, dates from 2006. I daresay a good new QR clamp would have been fine but I could see no reason to have one for myself.

I had noted that QR seems to be much more common on mountain bikes that road bikes. Perhaps people really do drop the seat before a rough ride but I've never bothered.

For my money they're less reliable than QR wheels. Generally wheels go into a vertical drop-out so they don't actually need a lot of force to hold the wheel in place - at least, with rim brakes.