I think the reasons why we don't use NPG outweigh the reasons for why we might. Pressurized cooling systems reduce the effects of steam pockets and cavitation and make the cooling system more efficient by raising the boiling point. And systems running without water have less heat transfer from engine to coolant than systems using water and anti-freeze. The effects of increased altitude are negated by the use of a pressurized system. It's not by mistake that both NASCAR and F1 place a limit on the cooling system pressure.