I do you carbon in my filteres and I've never noticed them doing any harm, but maybe someone knows something I dont, there are some myths though about carbon, for instance some people will go to major effort removing the carbon out of the filter prior to using medications etc, I've found this to be unneccessary, even if the carbon is filtering this out it'd be a fraction of the amount and the medication is still effective. However I do know that carbon is generally only good for about 2 weeks, so if you want to keep usin it then replacing it fornightly would seem to bve in order.
Personally speaking, i do think it makes water look better, but it can be annoynig to keep changing, I suppose thats the trade off.