I think a good way of finding out is examining where xenia colonies exist in their natural habitat. Protein skimming is mimicing the foaming of the waves which pushes organics to shore (one reason I don't go to the beach..haha). If they are found near the shores, they very well could be nature's filter for cleaning the water flowing back into deeper areas where SPS thrive. I don't know about phosphate being a source of nutrients for them, but I do feel strongly about nitrates and organics being a form of nutrient for them. Here's a shocker, my nitrates are in the 25mg/l range, and they're growing very well in my tank. Truth be told, even at near 100mg/l, they were thriving. I've often looked at xenias in tanks that are very "clean" and noticed that the xenias pulse faster and are alot more feathery. While they are very aethestically pleasing, I've always felt, that's their way of trying to gather more nutrients.
Another good example would be the Eco-sphere. If you look at the reasons why the shrimps do not reproduce in the Eco-sphere, is because they "know" if they reproduce, there would not be enough resources for them to survive. That is what's stated on their site.