This is all just my opinion but I think carbon filters are ok but the problem becomes that the activated charcoal does is not able to have an effect on a tank for very long since it basically becomes saturated and is then nothing more than a good place for bacteria to grow...which is both good and bad I guess. I do know that the charcoal is very good at getting any medicines out of the water after you are done using them and I believe that they can reduce your nitrates as well. Still, in my opinion, there are better ways to go about reducing nitrate levels such as water changes or getting some filter feeders (clams, scallops, some corals I believe) that will feed upon the nitrates.
I am of the opinon that it is never completely neccessary, in a non-emergency situation, to use any chemicals, or 'pillows' in any tank at all. My reasoning for this is that if you have a properly set up, stocked, and cycled tank, the tank should be able to deal with ammonia all on its own so no help is needed. In my 'common sense approach to logical thinking', using an ammonia pillow or packet is just going to hurt you in the long run since it will be taking away the benificial bacteria's food source which means that you cannot get any ammonia transformed into nitrites leading to more bacteria colony reduction.
Although I do see the need to use these in the case of an emergency or a dire situation in which you need to save some fishes' lives. Even then though, you could just be prolonging the inevitable and that is having a tank that cannot cycle.