I would say there is a third, 'balanced' category in the middle - those who shoot everything.
Yup. That's me.
If one shoots a big enough range of things a camera system with a very wide "shooting envelope" is a required part of the kit room, even if we have specialised equipment for specific things.
My main commercial work is people. For years I shot with Canon dSLRs until I moved to Hasselblad in search of better resolution and just-as-nice colours, I shot 100% with flash and leaf shutters meant a lot more to me than high ISO. The Hasselblad is great and I still use it in its niche.
I got itchy for different looks- bokeh-tastic f/1.4 ISO800+ natural light, for example. My old Canons could just about sorta handle that, but not reliably (AF was rubbish at low light levels, grain was ugly, etc.) I only have one Hasselblad f/2.8 lens, the highest ISO I really rate it at is 80, and I can't hand-hold it steady below about 1/125th.
Furthermore, I moved back to Wales and I wanted to try commercial landscape, for which I wanted light weight and high resolution. And before I knew it I was struggling to do astro-landscape photography with a Canon 7D Mark I (I'd sold my 5D). I also do jobs here and there as they crop up which means needing kit to cover a very wide range of shooting situations.
I also shoot video; a small cam on a lightweight gimbal is a very useful tool in the toolbox! Good performance at ISO800 is a real plus here too. I'd been using a Panasonic GH4.
So I needed something new to broaden my shooting envelope, and I chose the A7RII. I chose it over the dSLR options because of IBIS and lower weight, good-enough video, good high ISO, reasonable AF, excellent resolution.
It's absolutely turned into my go-to camera in the way the Canons used to be, which I
really wasn't expecting. I always used to take the Canon case with me on every shoot. Now I take the Sony case instead.
That's why I'll be adding an A7RIII next year,. I'll round out my lens lineup with the Sony 12-24 and 100-400, and adding the Sigma 14mm f/1.4 to improve the astro stuff. It addresses my main complaints with the A7RII. (Although the UI apparently is still pretty poor).
Canon could have sold me a 5D with dual pixel PDAF and 4K video with a decent codec, any time in the years since the 5DII came out.
But they've decided not to. The 1DXII is epic, especially for focussing in video, but just too heavy and not enough stills pixels for my main commercial uses. I've still got enough Canon lenses that they could just about grab me for one last hooray, but the 5DIV and the 6DII are just not competitive with the Sony option, for me.
Nikon could have sold me something if they'd come up with it sooner, although no IBIS is a bummer when you've got used to being able to handhold at 1/60th and get close to 40 megapixels of resolution out of it. If I didn't have Sony lenses now I'd be seriously looking.
I fancy the GH5 for video, but no PDAF, and there's just not enough resolution for me for stills. I find myself using the Sony over the GH4 more and more, not because it's better per se but because I've ALWAYS got the Sony with me on every trip. Add touch screen AF (big help on a gimbal, even if it is just to set focus at the start of a shot rather than tracking) and the A7RIII looks like a better bet. Plus I think the Sony lenses are a different class from the Panasonic ones I have (which is fair enough, they cost a whole lot more).
I'm tempted by the mirrorless MF stuff, and by a Hasselblad 6D, but I just don't think I can justify them when for me an A7RIII will fill so many more roles, and full frame 42 megapixels and f/1.4 lenses just cover more options than the MF 50 megapixels but slower lenses.
It all comes back to the "wide shooting envelope" and the cost of extras to get the system shootable. which is something Michael Erlewine touched on. It isn't the camera that costs the money as we all know, it's the need to invest in the system, especially the lenses.
So settling on one versatile body which can serve several needs is very helpful- it lets one leverage the expensive lenses in multiple different shooting situations, and conversely means I don't feel so bad about investing in the expensive lenses knowing I can always try them out for unexpected effects in different shooting scenarios. For example it turns out that my favourite landscape lens is the 70-200 f/4 Sony, which I totally did not buy for landscape photography!
Cheers, Hywel