My thinking is that if it came with the filter, it came with it for a reason. We are running Fuvals, as well as Undergravels filters on both the 30 gallon and the 75 gallon. The 75 gallon also has a third filter on the back, but I am too lazy to go look at the name of it right now. So the filters that came with carbon in them still do, unless we have another illness outbreak. As soon as it was clear the illnesses were gone for good, back in the carbon went.
Oh, we don't change the Fuval's materials that often. I mean, we change water, but depending on who you believe, the material can last longer between a week up to a couple of months. Now, maybe because we have so many filters, our stuff just might not get dirty as often.
We don't have live plants in my tanks, but do have drift wood for the many plecos. Also, we have a LOT of fish. So my Honey figured that for the sake of our fishes we best keep the carbon in to remove all the waste from the water.
The Tanks are clear as can be. The fishes are happy and healthy. Maybe they would be without carbon, but I am not about to change what is working fine.