It would be the easiest solution, but it would also mean that I couldn't have a monster spawner or anything like that nearby. For example, maybe I want the temple filled with skeletons spawned from a spawner, or from eggs, but without the risk of creepers spawning due to the darkness.
A bandaid solution would be to have it only stop creepers from spawning, or perhaps to give the entity a GUI, but that's still quite limiting. My intended use is for adventure maps, but also for survival. Imagine sticking a relic in your house and finally being able to switch off that shiny new redstone lamp. Dark rooms or moody lighting, without the risk of being blown to smithereens before you can say "Honey I'm home!" and hit the lights.
EDIT: Another problem with routinely deleting hostiles near the relic: what about mobs that are following you? would be rather silly to run into a room with a relic and suddenly all your pursuers vanish into thin air. ;/ Thanks for the suggestions, though, I hadn't thought of doing it this way before.