Heck, isn't it time we were honest? sdy284 did ask after all.
We don't use carbon in our tanks because we're broke.
After buying all the stuff for a nice pressurized CO2 setup there just isn't money left for filter carbon. It's easy to see the difference. We buy CO2 and have better looking plants. "They" buy into the carbon filter cartridges and suck perfectly good micro nutrients out of the water. Oh, wait, that's why we don't use carbon. Never mind the other stuff.
Ok, to give a more sage answer, Excel isn't carbon. It has lots of carbon atoms in it though. The active ingredient (which is a secret so don't do a search on the word glutaraldehyde) breaks down into bio available carbon. Some of the intermediary chemicals may be adsorbed by carbon. We'll need a chemist to chime in here.
If you do a search on the forum for carbon filtration, there are some good threads explaining why filter carbon does quite a lot of things. Some of the threads are even accurate. Not the most entertaining ones of course.
If that didn't answer your question, I can give you a different answer tomorrow.