The short answer is no, they don't need to add RNG to damage. The long answer is no because in most cases where there's a crit rate/crit damage stats, it's either insignificant to the point of being a waste of resources or effectively being damage% with extra steps where the "builds" are get to 100% Crate and then as much Cramage as you can stack. Besides that assuming they make it a worthwhile stat, if they increase boss health to compensate then it's entirely possible to fail a DPS check by pure bad luck, and if they don't then it's entirely possible to trivialize any boss through good luck making an inconsistent game experience, and in both cases making a crit build the best build.