I know everyone else has said that the fellow who told you that sitting idle will kill the plasma cutter is full of hot air... and they're right, of course.
However, just sitting around in a dirty environment can lead to an early demise. Where I used to work, we had a Thermal Dynamics brand "Dynapak" plasma cutter. It got used frequently in the fabrication shop and worked without a hitch until one guy borrowed it. When he brought it back the next day, it blew the circuit board the first time it was kicked in. Funny thing, he didn't even use it when he took it home. He just left it sitting on the seat in his pickup truck... with the window rolled down a little. It so happened that it rained that night, but not directly on the plasma cutter.
I tore into the dead unit to see what was up with it. It had blown apart 1 large capacitor and several of the Hexfet ICs. What was revealing, though, was that there were many places on the circuit board where very fine metallic dust had settled in. Remember, this unit spent its whole life in a fab shop which was also home to a drill press, a bandsaw, MIG and TIG welders as well as numerous grinding operations. Guess where the metal dust came from? Guess what killed the plasma cutter? I'm certain that some water got on the circuit board during the rain storm, pooled up with some metal dust and then slithered off to find high voltage to play with.
After that post-mortem, we decided to require the fab techs to gently blow any dust and whatnot out of the plasma cutters and welding machines at least once a week with compressed air.
To my knowledge, no more failures of that nature have occured there in the seven or so years after that preventive maintenance decision.
