Vince,
For me, the greatest advantage is lens selection, I've got about 14 SLR lenses, (primes and zooms) from 12mm at the short end up to a 300 2.8 (and teleconverters to take me up to 600 if I really needed it ), as well as a 105 micro-Nikkor, and I'd never have that kind of selection with a Nikon rangefinder, regardless of how much money I had to spend.
As for knowing if the strobe fired, yes, you are correct, with the rangefinder you'll see it but with the SLR you won't. So if I'm not sure, I'd ask the subject. Besides, only a fool would spend all that time and money setting up a shoot and then only take one shot of a particular pose or set-up. And as for the 1/60th or slower flash synch? Although my FA synchs at 1/250th, my F2 at 1/80th, and my F at 1/60th, there is no reason why they can't be used at a slower flash synch.
Everybody does what they do. That's Sadoff's Law. You do what you do and I do what I do. If shooting in a studio with a rangefinder makes you happy, that's great. Me? I'm just not convinced that there is any advantage to it, and your argument did nothing to change my opinion.