Maybe a better question to ask instead of what's the harm if nobody has been killed or harmed yet due to the ungrounded gear in their possession...would be why not take the high road ( ground..pun) and ground all gear? Thereby eliminating the one in a million possibility of death..particularly IF you happen to be the one!
component grounding
So this whole thing about grounding audio gear. There are millions of pieces of vintage audio gear out there without ground plugs. In fact, many don't even have a polarized two prong plug. Yet search as I may, I can't find one situation where someone was killed or seriously injured from an ungrounded piece of audio equipment. I've been in the HiFi business for over 50 years.....Never saw a recall or a warning or anything until recently. This past year I built a Bob Latino ST120 Tube Dynaco clone amplifier. I was surprised to see a lampcord power cord with no ground. When I inquired I was told by many to not ground it as it wasn't necessary and might introduce hum......Can someone show me documented case where injurie resulted from an ungrounded piece of equipment?