The rangefinder coupling simply means that when you focus the lens, the "patch" in the cameras finder will show when you are in focus! With ultra wides it is not critical, except when you are working at the closer focussing distances and close to wide open. Guessing focus below 1 meter is tricky - and, yes it does make a difference. "Scale focus" is fine, but at close up it will show if you are off even slightly.
With any rangefinder camera, you will need a top-mounted finder for field of view as nobody makes a body with a built in 15 finder (the R4 "only" goes down to 21mm). You simply focus with the cameras rangefinder, shift your eye to the aux. finder and "compose" and shoot (or the other way around, depending on your preferences). The M-mount 15 also has the advantage that it can take screw in filters, albeit large ones, 52mm diameter.
The focussing ability is more important with cameras like M8 and RD1 as the "crop factor" turns the lens into a 21 or 25 mm lens - and that is getting close to where "scale" focussing gets tricky.