A test ride can give you insight on how frame geometry might affect the way it FEELS when it pedals, etc. Like the curved seat stays on the litespeeds, you might be able to feel how they dampen vibrations or whatnot.
I didn't rely on fit as much as how my bike felt when I went over potholes, cobble stones, off curbs, and braking in the wet when I bought my commuter. These things, a fit won't be able to tell you. Besides, once you have a frame built specifically for you, you might realize you still have to adjust the stem, stem angle, crank, seat, etc, for fit.