Healers that don't want to DPS are like DPSes who refuse to move out of AoEs.

[Zojha]

First, you said "healer casually DPSing", not "healer DPSing in casual content"

Because there is no such thing as doing anything "casually" in non-casual content...? If you were doing content relevant activities casually, it would logically mean you're approaching the content casually, thus making it casual content for you. Therefore, the two are synonymous and preclude content you aren't doing casually. That's logical.

I'll also note that the DPS numbers for dungeons do not include trash pulls precisely because of the AoE. They're based on the boss fights. And I actually took the time to look it up for EX trials as well - In EX Trials, a healer doing slightly below average DPS only amounts to one DPS going from 50th to 90th - average. My, now you're only expecting "one" DPS to become pro to carry you instead of two. And improvement, sure, but...still unreasonable IMO.
(Oh, and for Creator Savage it's the same - White Mage aside. People are onto something when they say it suffers.)
Unfortunately however, 8-man numbers are a little skewed due to the whole "Main healer - Off healer" thing.

That said, you are certainly right that exceptional DPS can carry healers and tanks that aren't DPSing - When designing fights, SE assumes DPS will do 85-90% of their theoretical damage potential while tanks only spam the aggro combo and healers contribute 'zero' damage. So if DPS actually manage to do those 85-90%, tanks and healers could in fact not do anything of note - No Scourge, no Plunge, no Low Blow, no Dark Passenger, no Dark Arts, no Carve and Spit, no Darkside. Just aggro combo in grit and the occasional CD, while healers only heal. That's is possible by design and naturally, such groups will be able to achieve similar results to groups where tanks and healers actually contribute to damage but DPS are not gods of gaming. Personally however, I must say that I do not take this fact as reason to exempt tanks and healers from contributing, as it presents a huge double standard and sets unrealistic expectations on DPS, whereas expecting tanks and healers to contribute to DPS seems far more realistic.