Carbon has nothing to do with a wet/dry?
Carbon is a resin that pulls out toxins, certain nutrients, and dissolved organics..
I don't know what the reasoning would be for removing the carbon, but many hobbyist have found carbon to be uneffective as it once was thought.. Water changes replace the need for it, really..
Perhaps the discus keeper finds his Discus do better with no carbon? Perhaps the carbon pulls out a mineral that the Discus benefit from? I dunno..
A wet/dry will just help with biological filtration..
Pulling the 'charcoal' from your ray is up to you.. If you are doing the appropriate amount of water changes needed for a ray you shouldn't need to rely on carbon, because you are manually removing all those dissolved organics every time you syphon.