If you aren't a OCD reefer and want everything spotlessly clean, don't spend much time cleaning your sump. Some reefers let that silt build up in the bottom of their refugium or sump and never clean it out.
As for the sponges, I do everything I can to encourage their survival. Only a few sponges seem to survive well in reef tanks and they all tend to be ones that would rather grow in the dark like under rocks or in a refugium. I had an egg crate platform that held my rockscape about 1" to 2" above the sand. After 2+ years I broke it down and sold most of the livestock. The underside of the rocks sitting on the egg crate (above the sand) and the egg crate itself were loaded with sponges, small white feather dusters and other small critters.
I think it worked so well that I just did a new platform and rockscape under my 3 year old 50g cube. I gave it even more room underneath and created a 'cave door' opening at the front. I'm amazed at how much the fish swim in and out of the cave. And it makes for a ton of room for all the critters that like open but protected sand like serpent stars, flame scallops, small clams, shrimp, sea cucumbers and more. I just finished it and you can see it all in my build thread (there is a link in my signature below).
Good little filter feeders 
