But let's say you'd have a PLD who'd only Flash to keep hate (I'm only leveling PLD right now, it's 65, but so far seems possible), and a healer who'd only heal. You would be missing out on at least the DPS of one extra DD in the party, and leaving all the DPS on the 2 DDs. Would you consider that acceptable? Would it be somehow more acceptable for the healer to not DPS than the PLD, and if so, why?
To me, being a healer main from 1.0 until early SB (AST was my first 70), it's always been self-evident to me that DPS is part of my job, just like it is when I'm tanking, and I've never understood people who see it wouldn't be. And it's not about "dungeon taking longer" or not, it's about being able to be useful to your party - so why would you choose not to be?