You can't get the perfect AC with resist rares unless you have G20 resist blaze, G20 resist ice, G20 resist holy, G20 resist poison, G20 resist paralysis, G20 resist lightning.
I guess resist rares would be similar to WoW's resilience rares. AC is still useful for monsters or basic, none-skilled attacks like getting LB'ed by a priest staff or if players just don't use skills. Think how hard you hit with just 1 non-skilled attack with the target has almost no AC. So non-skilled attacks would be viable.
For warriors, we have multiple types of skills: lightning, darkness/fire, and wind/ice. If our target uses resist holy(wind), we can switch to lightning skills. If a priest has max resist ice and can't die to a thief, call an archer over who will do a ton of damage on the priest.
I think this will really distinguish the types of meelee classes as to what they can and can't do. I mean, after D1, thief and archer both use xfire/xice to PK. There's almsot no difference between the two other than a slight variation in damage/AC. You use the same strategy to fight both. Same rares to fight both.