Do I need 12" rear discs? Of course not. However, do I want them, sure. I do tow a fair amount and the additional rear brake capability can't hurt. I do have 4WABS and, as I understand the system, it will maintain the correct balance between the front and rear based on differential speed of the various wheels. I may still opt for the smaller v6/gt brakes. Bottom line is: I plan on discs all around. If I could install the larger '03+ front brakes without breaking the bank, I'm sure I'd go for the Cobra rear discs.
The avatar is of the aircraft I flew while in the US Navy, an A-6E, all weather attack. A fun time was had by all!