I know being a professional photographer is very competitive, but I don't know if camera choice is that relevant. At work, we regularly engage photographers for assignments. And I see a wide variety of equipment used. The majority are using 5Dii's and 1 series bodies. I saw my first 5Diii only a month or so ago. But I've seen rebels being used. There was even a bloke who turned up with a Nikon! Generally, when we're meeting photographers, just out of curiosity, one of the first questions is "what type of camera do you have?". I remember one girl who said a 500D. It raised an eyebrow, but a quick meeting made it clear that she knew her stuff and a car full of lighting equipment reinforced this. At the end of the day, we put our trust in the photographer to deliver. And a professional photographer generally will. To us, it more important that the photographer understands what we want than the camera they use. (....although...admittedly....if you have a 1Dx, Hasselblad, etc it does make a very good first impression).
We've used the Nikon guy for a few jobs. Shows how little the marketplace cares about the camera used.