Basically if the spawner spawns different amount of mobs, at different time in different light levels. If anything about the spawner was changed. You cannot use it yet as it had not been implemented in any snapshot, I think it's a planned feature.
I saw that, but didn't think it applied. Do you mean I would have to use the command on each individual spawner?
I think you would have to, yeah. I don't know of a way to execute it on all spawners in a world. If you haven't already, you might want to post this question on the redstone section of the forum, they're usually really helpful over there but the regulars don't seem to look at other sections all that much.