When coding a flash app, keep all interface elements externally as an imagemap(an external png with UI elements in it and an xml which defines the coordinates of ui elements in the png). Then reskinning would require only updating the imagemap rather than recompiling the swf.

By hiding the flash core ,basically they are exposing the flash apis to javascript .This would be helpful for frontend developers who are not comfortable with flash, but want to build a decent mp3 player .Other than the advantage for developers , i dont really think there is any functional advantage for the end user.