I think there are two reasons for not using a skimmer in a refugium-filtered system:
1. The skimmer may be counter-productive for the macroalgae, as Tanker pointed out.
2. Some believe that skimmers tend to remove a lot of desirable stuffs from your tank (i.e. trace elements, calcium, good proteins, etc.), along with the bad stuff. Thus, there's less of a need to add supplements and your corals will have more to eat, without a skimmer. Many skimmerless-heads still add Iron for the macro and a bit of calcium, etc., but report dramatic reductions in the amount that they have to dose without skimmers. As EcoSystem claims that their Mud containd all of the additives your tank will even need, I believe they suggest only adding a bit of calcium. I've got one EcoSystem setup, but don't know how much faith I have in a product named "miracle" anything.
IMOHO, the jury is still out on skimmerless refugium systems. I'm enjoying great success on two skimmerless sytsems so far, but it's still too early for me to recommend that anyone else give it a shot. Now, if I could only get that N03 to drop on my refugium-less system.
[ June 14, 2001: Message edited by: tangs_in_pacifica ]