A steel grate doesn't have much capacity for storing latent heat, instead it allows the pizza to bake by allowing the heat to pass right through it to the dough. A solid stone or heave steel deck can store latent heat and quickly transfer that heat to the pizza that has been placed on it so you actually end up getting a better bake if done right. Another way to look at it is like this;
You can put your hand inside of a 600F oven without getting burned...I do it all the time, just don't touch anything while you're in there.
Using that same oven and temperature, put a solid stone deck in it and allow it to fully heat to oven temperature, now if you were to touch that stone you would instantly get a burn. This is rate of heat transfer. Air is a poor transfer medium while steel, stone, water or oil are all much better and a lot more efficient. A couple of things that need to be considered with a solid deck, it must be sufficiently heavy to be able to store sufficient latent heat to provide heat to the bottom of the pizza during the entire baking cycle, and you must allow sufficient preheat time to allow the surface to absorb as much heat as it can (this could take the better part of an hour).
Tom Lehmann/The Dough Doctor