Both fears have some basis in fact.
Meggers can deliver harmful shocks. Even an annoying little zap can result in secondary accidents, such as falls.
A spinning motor without power is called... a generator! Sure, some motor designs are more efficient generators than others, but the effect is very real.
In a more unusual application, passing power through a motor has been used to correct power problems. Again, this is a highly engineered application, and isn't a DIY fix for power quality problems!
