Since the majority of climbing accidents occur on rappel, it seems like any discussion regarding why or why not to use certain techniques is worth it.
6mm accessory cord is not rope. It's not even really a "tag line". Accessory cord does stretch, but has neither the strength or durability of dynamic rope or static line. Sure, it's breaking strength is somewhere around 8kN, depending on manufacturer, but that makes the margin for error very small.
I think it's important to be totally up front about what equipment we use and why: rapping anything using 6mm is a VERY advanced move, requiring some honest discussion about the limitations of the equipment. The last thing we want to see is a bunch of people out there on rock routes thinking there's no concern about rappelling on their 9.2 with a length of 6-7mm accessory cord.