One good thing about a spare UNPLUGGED machine is in case of a lightning strike. We had a customer site get hit by or near by lighting. The unplugged spare wasn't connect in any way shape or form to anything. No power cord, no network connections, nothing. After the strike, they were having problems with their industrial systems and blamed it on the computers we installed. We took the spare system that wasn't plugged into anything, started it up, and things still didn't work. This proved that our system wasn't the problem. It turned out to be their hardware that didn't have adequate surge protection.