Why does it matter? And more specifically, what kind of pro? Are full-time artists pros? Or are you just thinking the stereotypical documentary/editorial photographer (a genre that has taking a massive beating since its Leica centered heyday)? If you have any cameras other than Leicas, does that disqualify you? I think you would not find many photographers who make their sole living with Leicas, though there are surely some out there. But it's kind of like asking if there are any taxi or limousine drivers who ONLY have a stretch limo and no other cars. Or an F1 driver who only has an F1 car. It's not that Leicas are so incredibly specialized, but more that if you earn your living taking pictures, you probably have a number of different tools, even if one or two are favorites. Same thing...if you earn your living driving, you probably have more than one car. And of course, the quality of the final image has only some relation to the camera and lens used...it is possible to make incredible photos with almost any system, as long as you properly manage its strengths and weaknesses.
Personally, I use only Leica for my digital cameras -- I have the S2 and M9, and I use them both in work and in a personal capacity. The S2 is great for art or landscape photography, and also in things like artwork reproduction, photographic "scanning" (for example of contact sheets or 8x10 film), and various studio work. In terms of sale of prints, most of my older work was shot on film, often on a Mamiya 7II or Hasselblad, but also on Leicas (including the newer digital ones). My work with the cameras is more as a sideline to my main business as a printer and lab owner, but I do consider a part of my profession, and at times those sales can make a huge difference in a month's business.
