cfsindorf

Why would you get in trouble? I think they are needed if you have a certain amount of bioload. For me, I liked a lot of fish and a mixed reef. Those two things make it hard to keep the water quality where it should be without lots of water changes. By running carbon and phosphate material through my own little version of a HOB filter type "reactor" it help me keep my livestock happier longer. And I stopped running carbon for a while. Once I started up again, I noticed a difference in water color and clarity immediately. It's an obvious difference

I found they are both very usefull. So I'd say a definite YES.
Carbon:
Due to clarity a nicer look. and healthier corals because of the better spectrum from the bulbs thus better growth thus better nutrient uptake.
Phospate:
The end of (hair) algea. Remove them once never to come back again. Less time cleaning, now more time enjoying the view.
Another benefit is that you can feed your tank more, resulting in lots of happy thankfull fish.
Be carefull though, if you're using it for the first time. Rinse every thing gently but thorough, start with small amounts of media and low flow through the reactor and ramp it up slowly as to not shocking your animals. ( bleeching )

What about on a fresh tank that has not had livestock added yet?

Craig

Fresh "saltwater" tank, right? There's no reason to run PO4 remover or carbon. Add those after the livestock is in.