I know what angle you are coming from Alvin... you are thinking, since the MS can just tune the injector pulse lengths, then what else could you possibly need? As boost goes us, just raise the pulse width, right?....

Here's the catch- by how much? Not only do you have to compute the additional fuel needed to combust the extra oxygen in the chamber, you ALSO have to take into account that the delta in pressures between the fuel rail and intake manifold is changing. A changing difference in pressure across the fuel injector makes the amount of fuel given at a certain pulse width different.

A 1:1 FPR makes the situation simpler. It automatically compensates for the raising manifold pressure, hence taking the manifold/fuel rail pressure change COMPLETELY out of the equation. it's done for simplicity.

If you had extremely high fuel rail pressures all the time, you could go without a FPR, if you were a genious and figured out the algorithm to change pulse lengths by "x amount" given "y pressure difference between rail and manifold"

hope this made sense