Anything metal will rust and release toxins. This won't be a huge deal if you change out the carbon filter every month and have sponges and other things in the filter so you don't kill the cycle.
I'm not sure how fast carbon removes toxins but I know it does. That isn't to say for sure that the fish won't be exposed to them every if there is carbon. Carbon also stops working after a month so it needs to be replaced that often for it to stay effective. IMO, that's just a waste of money. Carbon is basically useless if you don't allow chemicals to get into the tank in the first place.
I would keep some of those cartridges on hand, though, for emergencies, if you ever find that you need to remove meds or something that accidently got into your tank (like someone sprays air freshener over them). Carbon in the filter combined with a series of large water changes will remove those things. (A 100% change would probably work better but what if you notice it in the morning right before you have to leave for work or something? Plus, it will remove any small traces you possibly missed).
End rant, lol.