I think you are misunderstanding the feature.
This is something no skinner could do before. The term glow is being used, but a better term would be an overlay. The per pixel alpha graphic can overlay the original item (titlebar button, button, etc) and overlap other things.
So titlebar button overlays can expand past the windows borders. Thats the big thing, for mouseover effects you are no longer constrained by the size of the original control. You don't have to use if for glows as it could be used for tooltip like effects etc.