As a photographer who has used only Canon equipment in the digital age until recently, when I started buying Nikon equipment, I can sympathise with those who might have the dilemma of whether or not to switch brands.
Lenses are just as important as camera bodies. You need both to take a photo. If one has accummulated a few good Canon lenses over the years, it doesn't make much sense to start all over for the sake of an improved sensor in a couple of camera bodies.
However, if Nikon has a particular lens or two with more desirable characteristics than any Canon lens the photographer currently owns, and the photographer knows that such lenses are of a focal length that he will use a lot, then it might make sense to either switch brands or use both brands.
This was basically my situation. I wanted a good wide-angle zoom, and the Nikkor 14-24/2.8 seemed to be the best available. I bought my Nikon/Canon adapter first, then the lens which I used on my 5D for a while. Unfortunately, the adapter was not fully functional. There were too many annoying quirks, one of which was the refusal of the Canon 5D body to fully respond to the 'off' button when the lens was attached. Consequently, if either the lens or the battery was not removed after using the camera, the battery would go flat in a couple of days.
As a result of a certain degree of frustration with such quirks, I bought the D700 which I considered a worthwhile improvement in its own right, compared with my 5D. I had two reasons to switch to Nikon, but I continued to use my Canon 50D whenever I needed longer focal lengths than 24mm. I still use the 50D with my 100-400mm IS zoom, although I now also have a D7000 and another Nikkor lens. The extra weight of the 50D body seems fairly trivial compared with the weight of the 100-400, and the Nikon equivalent to the Canon 100-400 does not appear to be better, and perhaps even not as good.
I'm stuck with two systems, but don't think that I'm complaining.