User blog comment:UsoppThePotato/An idea for critical strikes./@comment-5266525-20131002222450

Critical strike is not fully random, in its defense. That mitigates most of the problems of RNG and means it really doesn't make a massive difference unless you happen to be two lategame ADCs dueling each other and the very few attacks you both pump out can conceivably have significantly different numbers of crits between them. Crit generally isn't worth rushing because you don't have the AD or AS to make significant use of it.

Incidentally, if champions DID somehow get enough of those stats to make it worth rushing then critical strike WOULD be much more toxic, which is at least part of the reason why Gangplank and Tryndamere are both so risky picks these days - their early laning relies on crits to be useful.