I can't think of a good reason, the rear brakes don't do much of the work so there probably isn't any reason to upgrade the rear rotors thickness.
That said, I don't know what thickness my '90 TT has in the rear, but I track my car with stock wheels and brake rotor sizes, the only upgrade I needed was to use race pads up front (Hawk Blues for me) and some DIY brake ducting to the front brakes. If your not tracking your car it doesn't matter for sure.