We had a lively debate over at http://lowcarber.org about the properties of the scum floating on top of stock and whether it really needs to be skimmed. I bravely volunteered to taste the scum during my next stock making, which happened to be the poaching liquid for yesterday's Succulent Chinese chicken. There were predictions it would taste off, bitter, pollute the soup with nasty flavors. I was concerned that it was perhaps a lot of nutrition being skimmed off.
At any rate, I skimmed it off dutifully and tasted it. Pardon the cliche but it tasted like chicken. No off flavors that I could detect however I'm not a super-taster. I am also not someone who is so revolted by the looks of something it translates into how I taste it (generally). To me, ugly food can taste good. I think I get this from my Mom through early childhood training. Our food was rarely pretty or especially appetizing to look at. Sometimes it was pretty awful looking and on occasion, it tasted pretty awful too. Mom was an experimental cook, as I am, but during a time when people first started experimenting with soy and stuff like that. I'll never forget those awful green soy/cashew burgers she made once.
The stock scum forms really early in the stock making process, like before a boil even starts so I doubt that it involves marrow and good stuff. It's just proteins that decide to clump together. Probably in the same way that beach scum forms. Perhaps it is the precursor to life! Amino acids bumping into one another, one day forming a simple organism.
At any rate the scum didn't taste off at all. No bitterness, no nastiness, just good old chicken flavor bundled into an unappealing looking gray, floating scum. Or I guess if we want to be more gracious we could call it "chicken foam".
So, if you're not overly concerned about the look of your stock, don't skim.