I certainly don’t doubt for a minute that O Sensei could have devastated his training partners. Their ukemi demonstrate their respect for that potential. But I’m told that Saito Sensei opined that without mercy, ukemi is impossible. Certainly that is not to imply that practicing severe forms and injuring people has a place in the dojo. Practicing the severe forms short of injury, however, may be fundamental. Without that, how can you personally claim to be merciful? You, whatever your mental state or intentions, would be constrained by your limited technical knowledge.
~ Charles Warren, from On Mercy