Over on the Blessing of Kings blog, Coriel makes an excellent point: does World of Warcraft need PvP-only or PvE-only spells? Why, you ask? For balance purposes. If some spells were lebelled as only usable in one type of combat or the other, it would prevent situations in which a sometimes helpful spell in PvE gets nerfed hard because it's too good in PvP. As an example, Coriel talks about the 2.2 change to Blessing of Sacrifice, which will give the skill a 1-minute cooldown:
It's [the change] pretty much aimed at PvP, where Sacrifice is constantly used to break Crowd Control for paladins. However, this change makes Sacrifice useless for both PvP and PvE purposes. Admittedly, Sacrifice doesn't get used an awful lot in PvE. But there are times when it is useful.
Perhaps it's time for Blizzard to take a new tact in its PvP balancing act and implement some PvP-only and some PvE-only spells -- preventing a PvP nerf from nerfing your PvE game and vice versa.