Here's how I use my Canons:
5D3 - high ISO event coverage, office shots using Speedlights, mixed lighting, studio portraits, fashion work, product etc
1dsMK2 - Long in the tooth, but such a robust camera - inclement weather out-door shooting, wet and or dirty locations (factories, wood shops etc) or anywhere else where the camera's weather sealing comes into play. It can really take the abuse.
And here's where the 3D would come in really handy - studio product, architectural, and in particular, studio dance or other action photography, allowing to shoot wide to accommodate for subject movement, cropping in and preserving the resolution. Assuming the Canon lenses can handle it..
So as a pro shooter, I could really benefit from a much higher resolution camera and welcome its arrival. The right tool for the job.