With the current way the game is designed there is no reason for Cleric Stance to be disabled in dungeons. If that were to be the case, we'd have a large amount of time where we, as healers, don't contribute to the fight. In Heavensward, for example, they'd have to dramatically lower the healing numbers or up the damage taken by players, neither of which are ideal solutions if you wanted healers to stick to healing. Many healers, myself included, don't raid but when I do my roulettes, if I have some downtime during fights I'm going to DoT and Malefic/Gravity when its safe to do so. Othewise, I'm spending 80% of the time doing nothing. This is how the current system is designed.

If the idea of making Cleric Stance outside of group content impossible to be used is implemented, you'd have to change the way battles are done in group content. There's simply not enough damage being applied for this to be the case at this current time.