Back in the days when there were more than 100 D1's on the road, and guys built them up, the addition of larger than stock tyres were thought to be reason enough to get vented discs. That, and extended periods of braking at low speed operation. You only do the fronts because the fronts do 60% plus of the braking and vented rotors are not available for the rears. I have Defender brakes on both of my '95's, I run 33's and operate in a pretty mountainous area, I think they're a pretty good up grade.