Intriguing thread....
So, I will throw my hat in the ring...
I shoot professionally. I shoot mostly people, some architectural and some products. I have two main kits, a Canon 1Dx with a plethora of Canon L series glass and a Phase One XF with a Credo 60 back.
I use both systems interchangeably but each has its strengths and weaknesses.
Canon 1Dx:
Built like a tank and completely rain proof. I have shot in the pouring rain. The camera uses a 18MP CMOS sensor. It has lightning fast multi-point autofocus that is near telepathic. I can move and my subject can move and I can move focus points and I can achieve focus in a microsecond. It is second nature with my experience and the customization that I have set in the camera. The system has produced beautiful, sharp, saturated, accurate photos repeatedly and is largely infallible as a system. However, it will only sync flash up to 1/250s. So, when I am outside and I want to use flash in bright sunlight, I can only go to 1/250s with my Profoto strobes. More on that in a minute.
Phase One XF/Credo 60:
Also built like a tank, literally! It has one focus point. One. I will say that again....one. Focus and recompose is a necessary evil. With the XF, the focus speed rivals the Canon. The one point, however, is a drawback, but, considering what follows, I can accept it. The digital back uses a 60MP CCD sensor. That is 3x the resolution, although I know you wanted reasons beyond resolution. But that IS, in fact, a factor in the decision-making for purchase. Using the output file from this system gives a better starting point for large scale prints. In addition, after personally comparing files, I have witnessed that the gradations from light to dark are more refined in the Phase One file than in the DSLR file. There are simply more tonality steps that stand up to more post processing. This results in smoother tonality and luminosity changes in light to dark transitions. Big points for MFD. The system syncs up to 1/1600s due to the leaf shutters not found in DLSR's and due to the speed of communication of the Profoto Air system. What does this mean? Well, if you are outside in bright sunlight and you want to shoot at f2.8 for an artistic reason, and use fill flash, and have the sky blue instead of white, you can. You can use all of the power you want from your Profoto strobes to fill in the shadows when you run the shutter speed up to 1/1600s. More power from the strobes allows them to be moved further from your subject which equates to more freedom to frame a shot. Can you do that with a DSLR? Sure if you want to gang together 4 or more speedlites. I choose not to and I get beautiful shallow depth of field, with great exposure of everything with little hassle. Big points for MFD. When I need to access a function, change a setting, or check an exposure, I use the two touch screens on the camera body and digital back. I am not pushing buttons and scrolling through menus like on the DSLR. I get to where I want effortlessly. Big points to MFD.
Both systems that I use represent, in my humble opinion, the best of their class (Yes, I know there are newer, better, backs). Both offer advantages and disadvantages and both are tools to achieve certain goals. Would I use the Phase system to capture a dancer jumping through the air with a streamer? No, I would choose the 1Dx and shoot 14fps with tracking focus. Would I use the Phase system to shoot a fabulous portrait or fashion shot outdoors in the sun with blue sky, shallow DOF and strobed fill on the model, you bet! When I print a 40" x 60" print for my studio gallery, would the Phase file print a little better all else being equal? Yup.
Aside from the performance issues above, do I like the build quality of the systems? Yes for both. Does that matter? There is a certain appreciation that I have for tools that are well designed, ergonomically efficient and that provide a great user experience. Is it any different from a mechanic that prefers to buy and use Snap-On as opposed to Craftsman or Harbor Freight?
In the end, the cost for entry for me was justified as a business expense with the benefit of providing a special look with the capabilities of the 1/1600s flash sync that the leaf shutters afford. You just don't get that with a DSLR. If you wanted to get that with a DSLR, it comes with more setup and time. And time, is money. The added resolution for the jobs that require the best resolution possible for large scale output is also a reason for the investment. And one last thing, with the XF's built in Profoto Air trigger, I simply turn on my camera and start shooting with strobes firing away. No trigger for the hotshoe. It just works.