The resistance calculation is very complex, check out kripparrian's videos on it if you want a full explanation but effectively perfect resistance rolls are still like 100x worse than average rolls on stats like damage reduction from close. For late game build optimisation, any resistance roll on any item is the worst stat you can roll. Coincidentally the two classes that build resistances off their main int stat, sorceror and necro, are apparently the worst performing classes in endgame