i have run it off and on. IMO...if your tank is very low nutrients, and very low bioload, dont run the carbon.
i ran carbon in my 20g nano last year when i got a little bit of a cyano outbreak, the carbon bleached a ton of my zoas and other corals. i had been doing weekly 5g water changes for about 6mos of the tanks life, so never had any detectable amounts of NH3, NO2, NO3, or PO4
i do run carbon in my bigger tank now. but only b/c i couldnt get rid of this hair algae problem (well, not really big enough to be a problem, but i'm a perfectionist...) but i still have a very low bioload, only 2 clownfish in 50g of water.
the carbon hasnt helped my algae problem at all. neither did phosban as i put both of them in a canister filter.
i also have assumptions that alot of the carbon like diamond, has phosphates in it and subsequently only add to the phosphate problems. and as you know with my phosphate kits, they are undetectable even if they are in the tank, due to the nature of how the phosphates are always bound to another chemical or used up instantly (i.e. there has to be free floating phosphate groups in order to produce a positive test result).