As pointed out in the -many- "to dps or not to dps" threads involving healers, healers do not have enough to do -always-. Even when content is new, that wiggle room exists, as it ages, players pick the most lazy and ineffective strategy (cast regen, do nothing else) because there is nothing else for that role to do, because the game makes no requirement to be reactive in anything but extreme/savage. It would be one thing if more content had "heal the idiot NPC" type of mechanics, it's another where "the DPS are taken out of the arena, and only the tank and healer can release them, simultaneously", and we just don't see mechanics that make the healer do anything but run away when targeted.
Like in a traditional RPG (eg pen and paper) your GM would adapt to idiotic and lazy gameplay. One way of making the Healer do more work is by attaching consequences to not paying attention. We see that with half dozen versions of Doom/Death. Some of the 24-player content is so busy that you can miss things, and I'd argue that the 24-player content is actually the best content in the game for healers, because it does not let you "do nothing", there is always going to be something that you can do because the mechanics often require paying attention to the field and other players. On the opposite side of this, most of the 8-player content has no balance once players have out-geared it, and it's a choreographed dance once you remember the how it goes. The 4-player content is a mixed bag, and as Yoshi-P stated, has no instant-death mechanics, so there is never any reason for a wipe other than people not paying attention.