Well, since BoS sticks a while, you will usually go into battles buffed. If you don't, there's probably a reason for that - Curse of Slowness, dispels - which are all single targeted spells. Being able to simply mass-BoS over them would make them much less useful. By having you single-target BoS (and other party buffs), you have to make a choice who you will be buffing in case multiple group members are affected.
