I don't get the obsession that amateurs have with 'justifying' a camera. If you can afford it and like the images you get with it, that's all you need.
Chris speaks the truth.
I personally find focusing my range finders much easier than my Hasselblad, I find I can only focus SLRs with a split screen, otherwise, I don't have much luck.
I don't "justify" range finders, as technically, you're probably better of with an SLR unless you shoot lots of IR or something, and looking through an IR filter does not appeal. I use range finders because I like them, end of story.
These days, DSLRs can have quieter shutters than range finders (try an SD15), and the size of an Olympus OM is comparable to a Leica, so size and sound does not make much sense to me as an argument for range finders.
What does make sense is how much fun they are, how easy low light focusing is (once you get used to it). For slow shutter speeds I like how the finder does not black out during shooting too.
But in the end, it's about like or dislike, not technical arguments. If you give range finders a go, and don't like them, fine, move on and find something else. Hell, if you like the idea of owning a Leica, but want an SLR, check out the Leica R series.