Manifold heat cross over physics and benifits

If blocking the crossover is so bad and detrimental, then Fel Pro would not sell the intake valley pan/gasket for the big block with the exhaust crossover blocked off, but they do. I have one in the box on the shelf in the shop. Now, as to the hows and whys, as I said, I am stupid, so I don't know all that. I do know keeping the air/fuel mixture cool on an HP engine is important. Seems to me blocking the exhaust crossover does just that, but again, I digress. I am no scientist.