[halb]

I think you worry too much. Microwave ovens are meant to be used. I see plenty of commercial ones with plastic floors and I have never seen scuffs and scratches to be a problem. The very old one I have at home (GE) has a glass tray but that's just to catch spills and make it easier to clean. Never used one with a turntable.

 

[halb]

Absolutely should be kept clean. I like them to be cleaned on a daily basis, more frequently if there are spills or splatters.

Failure to maintain the oven in a clean condition could lead to deterioration of the surface that could adversely affect the life of the appliance and possibly result in a hazardous situation."

It's most important to keep the seal area around the door clean because any buildup can prevent the door from closing properly and leakage of microwave energy can occur. Hopefully that's what they are talking about because otherwise I don't think I would want a unit that uses materials in its interior that could be attacked by something its meant to be used with.