I was thinking about this last night ( I have no life ) and I came to the conclusion that if the carbon removes unwanted chemicals, then surely it will also remove wanted chemicals too.
I asked a guy at my LFS
about it today ( I know, I know, but this guy is actually knowledgeable ) and he says they always stop running carbon for a week or so after a water change because it would make a dent into some of the trace elements before any corals etc could use them. Although this kinda goes with what I thought, I have no idea if it's true or not.
I do know that a lot of activated carbons will leach phosphates into the water, so make sure to buy a very good quality one that will do it considerably slower than a cheap one.
It seems like another case of the benefits far outweigh any negative impact, but I would definitely replace it at least once a month.