The bio bag that Vin's talking about does mechanical, biological and chemical filtration, so no, it's definitely not useless.
mechanical because it filters large particles
chemical because it has activated carbon which does 'clean' the water
biological because beneficial bacteria can grow on the filter floss
In my opinion, using activated carbon can be beneficial if changed out on a regular basis and I think it does filter the water of dissolved organics. It's my understanding that if the activated carbon is not changed regularly then some of the dissolved organics that it absorbed can slowly get released back into the system.
I know many folks advise not using carbon and I don't use it on a regular basis in my freshwater tanks. However, I do use it sometimes in my saltwater tank to improve water conditions such as removing dissolved organics that don't get removed by the protein
skimmer. This helps remove the slight yellowing of the water which allows better light penetration and a cleaner looking tank.
I think the problems arise when we forget to change the carbon. So if you do use it, be sure to replace at regular intervals. I don't think it will affect the cycle, whether you use it or not.