A strait blade fork using an offset crown restricts the framebuilder to just one pre-set fork rake, as determined by the angle of the offset cast into the crown. For a given style of offset fork crown there is usually only a single offset available to a small scale framebuilder from the supplier, not a bunch of fine graduations of available offsets. This may not be as big of a problem for a big factory building the exact same bike hundreds of times over if they can specify the offset as needed. But even then, different sizes of the same model likely should get slightly different rake for optimal handeling as the headtube angle is usually somewhat different for lage and small frames. If you still need to bend the "strait" blades to fine tune the rake, that sort of defeats the purpose of the cost savings of using strait blades.
By using a fork crown without offset and instead bending the blades, a builder can better fine tune the steering characteristics to suit the bike design, not compromize the design of the bike to fit the available castings. Bending fork blades to shape is suprizingly easy. I probably spent less than $20 on materials to construct a fork blade bending form and making the bends is really rather easy to do. I suspect that the one possible benefit of strait blades is that a builder might be able to get away using lighter tubing since it will not become weakened by the bending.