The main reason for the 4-cell GTB is size and weight, and it was optimized for the lower voltage from the 4-cell 1/12 scale cars.

The reason you would not need the XBR or standard GTB is because you don't run LiPo in 1/12 scale. The 1/12 scale layouts and rules accomodate for 4-cell only, 4.8V.