Why skim "scum" from the surface of a simmering stock?
Skimming is for aesthetic purposes.

The scum is denatured protein, mostly comprising the same proteins that make up egg whites.  It is harmless and flavorless, but visually unappealing.  Eventually, the foam will break up into microscopic particles and disperse into your stock, leaving it grayish and cloudy.  The more vigorously your stock bubbles, the faster this process will occur.  

If the grayness or cloudiness bothers you but skimming is not an option for some reason, you can always remove the micro-particulates later through the clarification process used to make consomme.