Originally, I wanted enhancers and potions to work this way -- like in Betrayal at Krondor/Antara, where you could prepare beforehand and the effects would last until end of next combat. That in itself raised the question of preparation -- if players could use them out of combat, they'd stack buffs and go barelling in, and then we'd have to balance around the assumption that players would fully buff, which would screw over those who didn't. By forcing combat use, it forces a decision to use your action economy in exchange for stacking buffs.
It wasn't exactly the solution I liked, but I don't really have that much say on the gameplay aspect of things.
