Carbon doesn't remove nitrates or phosphates, and unless it's got the added 'ammo-lock' granuales in it, it's not gonna absorb ammonia either.
debates abound as to whether carbon can absorb trace nutriets. I'm pretty sure it would absorb Excel though.
Biggest reason not to use it...is that its not at all necessary. Carbon's primary purpose is to remove odors, and remove medications. No one medicates constantly, so that rules out reason #2.
Normal water changes should keep a tank smelling clean, thus removing the need for #1.
Why spend the money and take the time to use it when it's not needed?