The question came up and I searched a few different phrases and found nothing.
I am curious if you or someone you know has switched slr systems from one brand to another and what was it that made you or them decide to do it?
Back in the 90's I switched from Contax/Yashica to Nikon's F-mount bodies, when I went digital initially I stayed with the F-mount and got a Fuji, I'm on my second Nikon body since then and don't see myself changing off of it soon.
I added Mamiya 645 and 6x6, Pentax 6x7, Calument 4x5 and a Canham 5x7 system to the mix a few years after going Nikon. Since I've gone digital though my film bodies have pretty-much just collected dust.
I get the shots I need with My D2x and I'm spending my money more on lighting these days than camera bodies. If I needed resolution or a larger sensor, I'd probably go with a 4x5 back or 645 back at the least.
My decision to go off of Contax/Yashica mounts was due to the high shutter speed of the Nikon 8008s body at the time and the direction my photography was taking at the time. These days, I'd switch without hesitation if I wasn't getting the shots I needed, but I am and I don't see myself even upgrading bodies unless the next Nikon body is significantly more compelling.
I just read the original question, so I'll expound a bit:
Most people switch because they're poor photographers and they think a different system will do them more good because of some angle or other that doesn't really affect the quality of *their* images. You can watch them on DPR- many of them go back and forth every few years whenever the next "best thing" hits until they learn they can't shoot with that either. Many of them seem to be one or two-person wedding businesses.
A few people switch because their current gear doesn't do what they need it to do (low light, a specific lens, the flash system...,) or because of handling issues (size, weight, etc.) Normally, they're working pros who can't afford to miss the shots their competitors are getting while their invested brand "catches up."
Many people keep multiple systems because of a particular feature set, lens or capability.
Finally, there are people who convince themselves they need something based upon the marketing hype generated by one or the other side of fanboys in the Nikon or Canon camps. That'd be the "full frame" fanboys who own only crop-factor Canons and the "metal body" D200 fanboys who've never cracked a polycarbonate body to illustrate both sides of the issue.
If you're a working pro, switching systems is a business decision and if you're not getting what you need to sell images, then you're going to ROI the new system pretty quickly. If you're not, then either it's because you're shooting in a narrow set of circumstances that don't apply to 98% of photographers, you're shooting with consumer lenses in challenging conditions, or you've overblown things in your mind enough to convince yourself that you are, or finally you're lazy (The Nikon Creative Lighting System takes the work out of shooting with multiple flashes pretty impressively, an extra stop and a half of high-ISO means you don't have to light things correctly...)