I think it may be the direct result of lowering bars of entry expansions over expansions.
Healers nowadays are so used to NOT healing. Whenever they make mistakes or perform abysmally bad, there are others that can clean up the mess. RDM and SMN can Raise. Tanks can sustan themselves for a long period of time and use their personal mit to save other teammates. WAR is invincinble. DPS has access to either high potency shields, HP regen, or HP restore.
No to mention healing requirement in casual content is too low to begin with. Damage interval from boss is long and slow. Healers can still get clears even if they completely disregard party's HP bar throughout the fight. Therefore, whenever situation arises, they're not ready to handle it. They don't know how to heal efficiently. They panic heal with Medica II when one dps has bleeding dot at critical HP. The envionment enables the existence of such healers, and I'm afraid there will be more of them in the future. Luckily, our loving Yoshi-p has forsaw it and deemed it neccessay to give DPS more healing tools to compensate in DT.