I think it is cheap insurance, personally. I have had things coated from PolyDyn(tops of pistons, piston skirts, bearings, etc) and I think it is very worth it; cheap horsepower. Like the others said, it keeps the heat inside the combustion chamber, rather than outside it. You can worry a little less about your EGT's, since you have the pistons coated. You could also get the top of your head(inside the combustion chamber) or the valves done too, while you have it all apart.
I had PolyDyn coat my exhaust manifold, turbo housing and downpipe, and I plan to do a lot more with them when I go nuts with my truck.
hope this helps