What you need, and what you want.
Having lurked in these forums [for years], I'll take this opportunity to post my current logic of the subject of the 'tools for the task at hand'.
It has echoes of the Original Poster [OP], and I'll summarize the experience backing the various logic.
I was taught photography by my sister, about two decades ago. At the time, she was a professional photographer, and one of the fundamentals of composition was that an image is an answer to "what an I seeing/ describing/ saying with this image". My background is very technical, being educated in aircraft assembly and working in the computer created visual field for over two decades.
Camera ownership history summary:
My first digital camera was a Canon S45, followed by a Fuji S2, and now a Nikon D200. My first film camera was a Nikon 2002, and now a Contax 645 [with a soon to be repaired Mamiya 645AFD]..
This thread [and the related first thread] is showing [and a summary of] the usual cycle of any new technique/ technology that replaces a formerly established form/ norm. Akin to the video [re]-evolution, many people seem to think that ONLY objective 'data' should be the criteria to judge and compare, among the new options, and against the superceded/ out-dated techniques.
There are numerous examples of 'revolutions' in visual production methods: desktop- publishing [from film to PostScript based], 3D animation [cell or stop motion to computer rendered], video editing [from 'A-B roll' to non-linear] . Many of these transitions had the new technique being 'objectively' rated as superior in EVERY WAY, while usually not being the case, at first.
I'll take the case of non-linear editing, because I was there from the beginning. Basically, producers [who adopted the new editing techniques] lowered their quality standards to gain a massive increase in production speed and many more editing options, compared to what they replaced. I have made two identical edits from the same source material [as the editor], and the producer showed our high end client the results, blind. The client could, without exception, pick out the non-linear edit, every single time. It is only when the non-lin systems matched the prior system [in visual quality] that the client would accept the edits as final. You need prior experience in a professional setting, with experienced clients, for the quality standards to hold.
And we are seeing this in these Lu-La forums, again.
The reason I own and use medium format, with slide film, related to an experience being the bit-wrngler on a big corporate photo shoot. The professional photographer has been advised that the photo shoot was being arranged with many 'stars and planets aligning themselves' , and that the best image quality would be appreciated.
There are two general scenarios that the photographer could choose, both described/ discussed by BCooter in the original post:
"That is the nature of our competitive field and though I and many others could probably shoot most projects with a 5d2, cheap photo floods, and/or off brand strobes, there is just too much at stake to add risk using any marginal element."
- Photographer uses full frame Canon [which they own], getting the job done with decent and acceptable quality. To be clear, the photographer does not loose the client using the [full frame] Canon. There are no obvious issues, quality-wise, to using a [full frame] Canon,: that is the photographers' main camera system.
"As we all know a different camera won't make you a great photographer but, if your serious about what you produce, you'll be serious about what you produce it with."
- Photographer rents Hasselblad H1 with Leaf Valeo 22MP back.
Photographer chooses (B). As the bit-wrangler, I have to use the system to be at ease on the set, so the system is dropped-off at the photographers' studio the day before. I will note that I was very skeptical of any great difference in quality between viable digital cameras. I let it be known that many questions would be answered by my own testing as it pertains to ultimate, overall quality. This is when all the internet 'facts' would battle with actual experience.
After a few hours of trying to convince ourselves that medium format is [mostly] hype, MY conclusion of MY testing had me promising myself that I would move to having medium format in my stable of cameras. The results are "incountournable": I could not get around the fact that medium format could, and did, deliver visibly, and predictably better results. Period.
Which is why I own a Contax 645 system [with film backs]. It is the progression towards a digital medium format system.
For me, the data points me to a predictably better image quality over the 35mm system, with enough of an improvement to convince me to invest in such a system.
To each their own.
But to me, this internet discussion forum 'talk' is always weighted with my current mantra: there are no facts, only data. Interpret as you see fit.
One does have to convince others. On the internet, there are no unassailable facts.
It has been iterated several times on these realted theads, and I add my voice to the basic notions that I went to for my conclusions: make your own tests. Accurately compare. Own your decision.
[I'll try and keep it shorter, if there is a next time.]