Plate Warmers - whats the point?

Why spend a ball of money on Plate Warmers when all you have to do is whack 'em into the oven and two minutes later, Bobs your Uncle.  The problem is that for a few bucks you are risking damage to the magnetron.  Ceramic plates can have as little as 3% water.  That's not enough to absorb all the microwaves.  Microwaves density will increase rapidly.  They can then make their way back through the wave-guides and damage the magnetron.  This will cause the power to drop over time.  Hot water will do the trick but cost a fortune in hot water.