In planning out my brake system, I found that the proportion valve for the rear brakes might be a waste. In my experience with my last T, the rear brakes never seemed to lock up. And on a T with disc brakes up front and drums on the rear, the front brakes are what actually stopped the car. Maybe too much. That may explain why Spirit installs one in the front line. This time around, I may not install one. Of course, another thought is, considering the low cost, why not? That's also my thoughts on residual valves. Low cost insurance. And as Fletch stated, when I'm done with the brakes, I want to be done.
Also, in reading the responses, it's as if each installation yielded various results. Some needed PV's and RPV's and some didn't and every combination in between. Logically, if you have it in and don't need it, no worries. Unless they leak. Remember, in a brake system, the fewer the connections, the fewer chances for leaks. But if everything is installed without and you feel you need them, there you are taking your system apart. Hmmm..... much to consider, as I move forward with my car.