I think the idea behind the thread is that all roles have been given so many survival tools that the healer's 'niche' is basically superfluous in the majority of the battle content, with the hopes that either decreasing those tools effectiveness or increasing the average unavoidable damage across the board that this situation would make healer a good choice.

It's been a pretty steady decline of incoming damage since like 3.4 where they consciously tried to make extremes and savages easier, and a steady increase of a healer's tools since HW when they introduced AoE instant heals for all of them. It didn't help that they shifted so many of the damage mitigation tools off of healers to other jobs in StB, but people were solo healing a12s at least before that. Not sure about a8s when it was current, that would probably be tougher.

As it is, the main reason to bring a healer to most duties is that the queue requires them. In Extremes and Raids (and even Ultimates) the main reason is that certain mechanics target two healers, and the encounter is more predictable as a result. The central idea is that the reason to bring a healer has not been 'incoming damage is not survivable for a party without one/two healers.'