Two arguements can be made
1) Afaik we don't know the formula being used to calculate healer damage exactly, who's to say it doesn't account for the gain you would have previously had in cleric stance?
2) Let's say it doesn't account, we could also argue that you'll gain dps in effective up-time on the boss due to not being locked out from healing or dps ever. Being able to seamlessly transition from healing to dps without needing to worry about a stance or it's cooldown (albeit a low cooldown)
