Skimmers and fuges offset bioload. If you don't have a large bioload you don't need either.
Not a fan of keeping rock, or biomedia or anything not necessary in my sump because it falls along the same fail logic as bio wheels and wet drys. You want your biological filter in the tank...not in an external filter assembly. I have the same contempt for keeping bio rings and other media in cannister filters. Completely and totally doesn't make a bit of sense. You aren't solving a problem. You are creating a potential one.
Some reefers keep rubble for zoas or softies in their sump and use it as an ad hoc grow out. That's fine.
Just throwing extra rock in there though traps debris and creates another source of nitrate. It's just something more you have to clean, collects deitrus, and competes with the biological filter in the tank. So, I prefer to keep sumps simple and doing a specific job.