You can't just remove the piety buff. It is still technically a primary stat, in the same way that vitality is, needing to meet thresholds to get through content making it very valuable up until you have enough at which point extra becomes useless.
LDR had the right idea in that the buffs shouldn't be tied to specific classes but triggered by varied jobs filling the roles. So long as a healer is present the mind buff should be present, and so long as 2 healers are present and not the same job then the piety buff should be active. All the bonuses should be based on filling the roles with varied jobs rather than tied to a job specifically so that when adding new jobs to the game you don't have to try and shoehorn a bonus on them.