Sand or mud just adds places for denitrification to occur. Also might provide some habitat for pods and other zooplankton, but by no means neccessary.
Mud I think over sand is more used for growing macros in. I could be wrong, but you dont really need the mud unless you just want it or you want to grow macros that perform better in it. Most macros we typically use for nutrient export dont need the mud, and not even the sand with the exception of maybe some types of calurpa.
FWIW (and maybe not applicable here) I tried just putting live rock rubble in the sump and didnt like it. THe problem was that it just trapped detritus, but didnt have the ability that sand does to properly denitrify it. It would be easier imo if you were going sandless in the fuge to have only a few big pieces of LR as opposed to rubble that you can remove and/or siphon around. The rubble was impossible to clean around, and by rubble I mean 20 lbs of nothing bigger than a golf ball.
