think its pointless on MFI, if this was not the case why did ford not put one on?

It induces an air leak into the car, and you can prove this as not all dump valves will work, put a diaphragm dump valve on a RSTurbo and the car will idle very badly.

As for prolonging the life of your turbo, where is the evidence for this ?
I also doubt that most RS turbo owners run enough boost to consider compressor stall to be a major problem.

The real deciding factor imho is do you want your car to sound like a bus