I agree that poor belaying technique in general shouldn't be accepted, if for no other reason than that it sets a bad example. That being said, the pro climber

I agree that poor belaying technique in general shouldn't be accepted, if for no other reason than that it sets a bad example. That being said, the pro climbers can probably catch the non-brake strand of rope with their bare hands (they're strong enough to grip the rope with their full weight on it), so if they have a hand on the rope at all, they're probably fine whether or not the belay device is catching the fall. There's a famous situation (https://archive.is/tHQh0 https://archive.is/tHQh0) where Alex Honnold and Emily Harrington (another pro climber) were starting a simulclimb (a type of climbing where two people climb at the same time). Emily Harrington was on the wall and getting to the point where Alex would start climbing. The rope was going through his Grigri (auto-locking belay device). But Emily fell far above her last piece of protection, and the rope started to pool on the ground next to Alex's feet. Normally a climber would fall at least double the distance between them and their last piece of protection, so if you are 2 meters above your last piece, you may fall anywhere from 4-7 meters before stopping (rope stretch and the amount of rope and slack in the system are additional factors) When you fall, the rope also falls, until you get to your protection and continue falling, after which it pulls the rope back up. The fact that Alex was able to react to the rope pooling suggests she was falling a pretty incredible distance, I suspect at least 5-6 meters, before reaching her piece. Had Alex caught her with the brake strand, she may have fallen up to 3 times as far, possibly hitting the ground and sustaining much worse injuries or death. Instead, he plucked the rope out of the air so she fell a much shorter distance, and caught the fall with his grip strength and body weight alone.