The only compelling reason I would have to choose any rangefinder over an SLR in a formal portrait studio would be for the quality and character of the lenses I had for it. Depth-of-field preview alone makes an SLR stand out in this regard, for me, at least. The framing issue, as well, though I've always tended to crop the hell out of my portraits. With that said, Our Man In Milan above has shown that a pocketful of Summicrae and an M-mount body aren't necessarily a handicap.
I'm planning to shoot a local writer's "author photo" next month, and for that mixed indoor/outdoor candid shoot, I plan to use the CL, the 40mm Summicron, and the 50mm Summilux. Less bulky than my SLR, easier to hold slower speeds with in the dim light I plan to place her in, and way better glass than what I have in reflex right now.