Nope. The point is that no current flows through SW1A, so the contact resistance is unimportant. The full sense current flows through SW1B, and thus it has some voltage drop due to contact resistance. If you omitted SW1A, the sense lead would see the voltage drop across SW1B in series with the sense resistor. It would cause a serious error on the milliamps range, a non-negligible error on the microamps range, and no error on the nanoamps.