I use carbon 24/7 and here is my reasons.One,My water smells more fresh.Two,in my mixed reef tank my corals are having a chemical warfare against one another.Carbon removes that but so does other chemical media.Here's the bad they can leak nitrate/phosphate back into the tank if its cheap carbon that does not sizzle when ran under tapwater.I replace mine every 2 weeks so this doesn't happen.
Carbon does also remove trace elements,especially iodine but I dose for that.I never seen my calcium get low though from using it.Some people run carbon 24/7,some part time and some people don't use it at all.It works for me,been doing it for too many years to stop.
Remember its a choice and subjective.