The inevitable issue with random mechanics with high damage is that it introduces an amount of uncertainty that leads to choice paralysis. If I DPS now -- because I know there's a chance there's nothing going on -- I'm going to be contributing more to the completion of the instance. But if I'm in stance and something happens (for example if I just turned it on) everything could go to hell. This sort of thing already makes some healers nervous in the first place, and increasing the output of burst or random damage only makes it worse.

If the meta is damage and all tanks and dps follow it (at least that's what I've mostly seen from more competent groups), then healers slipping into it too isn't necessarily a bad thing. Squeenix can either shift the meta away forcibly (ejecting those who enjoyed pushing damage to make a subset which may or may not be able to handle the healing requirements), or embrace it and make it so that DPSing is easier. Sacrificing everything in the name of DPS is considered pretty bad in a lot of circles (casual players, this forum itself) but it's the simplest point of optimization that gives people an easy-to-see numerical goal. The amount you heal will always be capped off by 'nobody died' and fighting down overheal.