It's just unnatural and goes against everything a fighter is trained to do. If a guy is clear headed enough to understand the situation he wouldn't really be that hurt but it dies happen on instinct. It can be dangerous and iffy, Mathis jr did it vs Bowe and got cold cocked by Bowe basically ending the fight. Some guys lose the count. Years ago it happened to Rafael Ruelas vs Martin Quiroz...his corner was off count with refs and just blew it. Oddly...an iron chinned Mercer took a knee vs Holyfield even though being hurt and on the deck was very very foreign to him.