What is the difference between:
A) PLD doing 100 DPS, DRG doing 350 DPS
and
B) PLD doing 150 DPS, DRG doing 300 DPS

Trick question, there is no difference. You end up with 450 DPS both ways.

Arguing that tank DPS "doesn't matter", for any reason, is lazy. You hold DPS to a high standard, and holding good healers to a higher standard (SCH DPS) is becoming more prevalent. Are tanks exempt because their job is "so hard"? (Spoilers: It isn't. Tanks are super easy crayon-drawing mode.)

Dying as a tank only happens for 1 of 3 reasons:

1) You forgot to pop a cooldown for a tank-killer. Your fault.
2) Your healers sucked. Healer's fault.
3) Mechanic wipes you. Somebody's fault.

None of these are gear-dependent. Parry doesn't help any of these.