Getting hit by avoidable mechanics is part and parcel of learning an encounter. Making errors is just a basic part of being human. Being ready and able to heal through those problems enables seeing more mechanics and learning. Being ready and able to heal through those problems helps to otherwise wipes into clears.

While this is especially true at the extreme/savage+ tier, it should be kept in mind at all levels of play. Every encounter in the game is designed around the idea there will be some degree of error in execution and that a large part of healer contribution will come in correcting for those errors.

There is a reason why the game calls for two healers when with exceptional execution almost everything can be solo healed: The baseline assumption even for the toughest content is not perfect or even great execution. All other issues aside, the idea that healers should only heal avoidable damage and that doing otherwise somehow constitutes coddling or poor play is farcical to the point of bordering on self-parody.