On other fights, you want more cooldown power and ability to stack cooldowns together. AG synergizes very well with Ascendance, because both are generally maximized by having Healing Rain down before using them and then spamming Healing Surge (With the CH buffs, it will probably be spamming CH instead next patch). Using both of them at once lets you burst 5 million+ healing, which is like having a second Tranq type raid cooldown.
I really don't think that not wanting to have another keybind is a legitimate argument for always taking RS, even when fight mechanics clearly favor AG. No offense, but that is just laziness and bad play.