I thought this would be about the difficulty focusing wide angles on slrs. That is the Achilles' heel of slrs to me, and also the place rangefinders really shine.
For the rest, I'm in agreement with AgX that the retrofocus design can offer some advantages. Still, there are trade offs. The best modern slr wide angles are supposedly fabulous, but are generally much larger than I would want. Rangefinder lenses are usually a much smaller package.
BTW, many rangefinder lenses shorter than 35 are retrofocus too, but still shorter than slrs. Leica started doing that in the 70's, I think, to allow clearance for metering.