I think there are plenty of stories about famous Leica photographers who used Nikons or some other SLR at various times throughout their careers. For street photography even.
In NYC, I have found people often to be more amenable to being photographed when I am using a big "pro-look" SLR, because they are all itching to be famous or show up in Bill Cunningham's street fashion features or are just plain vainglorious. Of course, with the Leica, I can often take their photo without them registering what I am doing, but I can do that with my Canon digital P&S too. Sometimes though, the SLR makes it more fun. Just sayin'.
My wrestling match is whether a D700 (I have a ton of ancient Nikon fast primes, like a 58mm f1.2 and similar, and the Nikon F4 is my favorite SLR) or a Canon 5DmkII (it will do Nikkors fine with an adapter and stop down metering, & what a great sensor in that thing!)
I also like the 5D's movie quality better. 🙂