The shield gas (typically argon or argon mix) on your mig is there to protect the weld from the effects of surrounding air (primarily the oxygen component). The air (or inert gas nitrogen) in the case of the plasma cutter is essentially there to "blow" the molten metal out of the cut. Don't throw away those grinders though. If you free hand a plasma cut you'll still need to do some edge dressing, just not as much as with a torch or whatever other butchering implement you're used to using.