You mean
-) fast lenses make only sense with SLRS, due to the brighter finder image, nonwithstanding effects on shutter speed or DOF?
or
-) fast lenses make no sense at rangefinder camera due to the rangefinder not being precise enough?
I mean the nature of SLRs is that you view at full aperture*. This is generally held to be a disadvantage for composition**. However if the lighting conditions necessitate the widest possible aperture, being able to see how the scene will look photographed is a definite advantage***.
*Some SLRs allow you to view stopped down, with the disadvantage of a darkening screen. This makes focusing more difficult, or a multistage process of focusing, stopping down, opening up, re-focusing, etc
**A rangefinder or Galilean viewfinder allows everything in the frame to appear sharp. This is a useful, even vital compositional aid especially in rapidly moving arrangements of people. At wide apertures it can be a misleading representation of the scene as a whole.
***Depending on the maximum aperture of the lens, and its focal length, a reflex view may not be a disadvantage. For example my 28mm Nikon 3.5 AIS shows everything sharply. More difficult to focus precisely, but the DoF is so wide exact focus is irrelevant. Even my 28mm 1.8 has a focus ring shift of a couple of degrees between infinity and 5 ft.