In the editor, there are two ways to attach an image. You can just attach them to slots, or you can attach them to skin attachments. If attached to a skin attachment, when exported the attachment goes in the skin. If just attached to a slot, when exported where does it go? If it went in the skin, then it would have to go in all the skins. So instead it gets put in a "default" skin. The idea is that the runtimes check the skin for an attachment (if set), then if not found check the default skin (if set).
Thinking through it all again, this may not have been the best idea for the runtimes. It'd be better to have only one active skin to check. At load time the default skin could be copied and use as a base to load the skin attachments in. The default skin would appear in the skin list as it does now, but wouldn't be stored specially in a field.
What do you guys think?