The liability I'm referring to is when you swing and miss and the keys come back at you. Swinging a weight on the end of a rope is a weapon that's been around since the Chinese started using so called 'peasant weapons'...likely earlier than that in fact. I've tried training with a rope dagger and when you screw up you're in trouble...and that's where the training comes in. The same risks are present if you had a roll of quarters in a tube sock...just not as sharp.

You're 100% right about the knife too...without training it's a far less effective weapon and can be a liability as well. I've never trained to fight with a knife. The closest thing I've ever used in my Kung Fu experience is a Butterfly sword which is akin to a meat cleaver (another peasant weapon). That being said I'd likely still reach for my pocket knife if I thought my life were in danger.