I'm fairly new to the hobby so please feel free to correct me if anything that i say is wrong but I would think that it all depends on what livestock you plan on having.
Pistol shrimp, watchman goby, sting rays, nassarius snails ect. should have a sand bed 3-4 inches deep. Some people say that deep sand beds hold toxins unless you have live stock like listed above that help keep the sand clean my moving it around.
Some people don't have any sand in there tank (i found people that have only coral tanks do this) and they find it is easier to keep there tanks cleans, keeps there parameters in line, and no risk of the sand beds releasing toxins that will kill all there corals.
No sand will make it harder for detritus to settle and become trapped on the bottom. Good water movement will keep it up and in the water column and be able to get mechanically filtered out...
Deep sand w/o critters to stir it up, stars/snails, can trap pockets of ammonia build up... Would I really be overly concerned with this.. not really.
I personally dont care for the look of no sand and conversly dont like seeing the wall of sand at the side of the tank. I keep 2-3"... never a problem with ammonia and the fish/currents move the sand where they will anyway.
