How can tanking and healing be made more broadly useful in more "role-specific" ways?

[Shurrikhan]

Extending the maximal encounter time, that's the big difference.

Sorry, what I meant to ask is what's then changed as to a healer's purpose? You can have an arbitrary time limit creating a set maxima, wherein healers increase maximum encounter time, or a slow descent which produces a scaling maxima, wherein healers increase maximum encounter time. I'm just trying to figure out where the actual change is occurring to how a healer would fit in, in its own feeling of play or as a piece of the larger puzzle, so I can list it.

Would I be correct in thinking that the idea here is that then healing, too, can contribute against DPS checks (by extending the window), in an equitable but distinct way from dealing damage?

If so, on paper that seems pretty neat, pretty solid, but in actuality, I feel like to judge or envision the idea fairly I'd need to know where's the threshold, and what does progression by skill, over the given fight, or over gear look like? Or, more importantly, what healer "throughput", exactly, would the boss be gradually overwhelming? Cure II spam? Cure I? Highest possible burst? HoT maintenance + C1 + periodic Benison? And how would avoid the same issues of "any healing beyond what's necessary to kill it in time is excessive", or gear turning a highest direct heal global + HoT maintenance into alternating highest direct heal with best filler attack + HoT and DoT maintenance? Wouldn't that actually allow for healing to be devalued even more quickly than now? No matter how I run this through my head — and this is probably just my idiocy, but I really can't imagine otherwise — it just feels like the same, but where you spend a long time ramping up to and a long time exceeding what would feel like the intended pace of a fight (or, its healing or mitigation requirements, assuming mitigation were ever on demand rather than just on cooldown/counter), with less interaction with the fight itself.

Sorry, I'm just trying to understand this better.