Those things aren't really comparable. You're right that Parry doesn't really do anything for big single attacks, but parry's definitely worthwhile during a big dungeon pull or any time you're tanking multiple enemies, because that's where average damage dominates. That's also the sort of situation where extra DPS stats are the least worthwhile (at least as a PLD).
I think you're looking at it from the wrong direction. You don't stack parry because you're expecting a single parry to make the difference between life or death. You stack parry because the benefits of stacking DPS stats are extremely marginal for a tank in tanking stance, and parry decreases the amount of damage you take over time.
And a healer most certainly can underheal in anticipation of a block or parry. It's called staying in cleric stance and using abilities like stone skin, lustrate, benediction, regen, and so forth as the primary means to heal the tank rather than relying on cure/physick. If you take less damage over time, that becomes easier to do. Obviously they still need to top you off before big hits and that sort of thing, but it's not like there's zero benefit to increasing your mitigation over time as a tank.