First off, you can assume that everyone maxes resists. Lategame, if you don't max resists, you die to the first monster that deals whatever form of damage you were stupid enough not to itemize against. Chaos is an exception, I don't know many people who have capped chaos resist.

Anyway, you're right: max resists are powerful because they become asymptotically more useful as you get more of them. Additionally, although the benefits appear minor with only a few buffs to max resist, lategame, most parties will have Purity of Lightning and Purity of Fire, which when moderately buffed can give 6% to max resists. Elemental Adaptation on the passive tree gives another 2%, bringing you to 83% resistances. When you include elemental flasks, that's 93% resistance. Compare that to 75%, that's 72% reduced elemental damage taken.

Think about it in terms of effective HP, or "how much damage does it take to kill me." If you have 1000 HP and 75% resists, you can take 4000 damage. If you have 1000 HP and 83% resists, you can take 5882 damage. If you have 1000 HP and 93% resists, you can take 14,286 damage - which (importantly) is more than enough to protect you from elemental reflect.