Using a crop or whip would be last resort obviously, I meant that even just having it and using it an extension of yourself helps loads. But if it came down to it and a horse was getting seriously dangerous (literally going to run me over, kick me, bite, etc) I wouldn't hesitate to use a crop or smack them with a lead rope. I'd never hit a horse with a lunge whip. But a crop makes more noise than the pain it causes (if any) and could save yourself from serious injury. Just saying. No one's saying to go in there smacking all the horses you can. Just that you can't be afraid to give a smack where deserved. In the right place, at the right time.
I give myself very good advice, But I very seldom follow it