True enough. I especially favor your last two paragraphs.
There are two reasons to need interchangeable backs: Quick film changes, and switching between different films. With a 645z, the latter is just a few button pushes. Want to switch from ISO 400 to ISO 3200 for a shot? One button push and rotation of the thumb dial. And popping another memory card in there every 700-800 shots (I use a 64G card) seems no real heartache.
But even with the film 645, which I used for weddings, switching inserts is no more difficult than switching backs. In fact it's easier, because you don't have to worry about that thin dark slide. I own four 120 and two 220 inserts. That would cover me for the pre-ceremony batch, and then I would reload during the preacher's homily. Then I'd be good for the altar returns, and then I would reload after the B&G had arrived at the reception. Then I'd be good for the reception activities and escape.
Of course, not all the Mamiyas had interchangeable backs.
But lots and lots of pros used Mamiya C330's for weddings, too, and those certainly don't have interchangeable backs. You learn how to load film fast.
Did you ever switch from black and white to color? Me, neither. Changing film speed was more useful, but I had two 645's, and often would bring my Exakta 66, too. I would set the Exakta up in the balcony with a fisheye lens to get a comprehensive view of the church interior, and maybe a comprehensive group shot, looking down on the assembled crowd from a ladder (that worked well for small weddings). The N would have the 75LS lens on it, with a QuickFlip and a Vivitar 283 plugged into the lens. The NII would have the 45-85, with the AF500FTZ.
Polaroid was another advantage to interchangeable backs, but not any more, and never at weddings.
Now, for the leaf shutter thing. The reason Hasselblad went to leaf shutters is because the focal-plane shutter in the 1600f was too unreliable. But now you have a different shutter in every lens, which means a different meaning of shutter speeds in every lens. If you just use one lens, you get used to it, but if you change lenses often, you end up with a table of corrections on the side of the camera. Nobody I knew would send their lenses in for CLA every year or two, and even then +/- a third of a stop was acceptable variation.
Now, of course, there is high-speed synch, and the two leaf-shutter lenses. I would only use those for set shots, though. In an outdoor venue, I'd rather have the assistant holding a white reflector than a second flash on a stick as is now common practice. But it is a real limitation, to be sure. Most photographers these days use small-format cameras, and they only gain one stop of shutter speeds for flash sync (1/256 instead of 1/125). I've had HSS on my Canon for years and I'm not sure I've ever used it.
And it only takes a 2x neutral density filter to be able to run f/8 with a flash at iso 100, even in full sun, with a sync speed of 1/125.
Part of the appeal of Hasselblad, Mamiya, and Bronica was that by the time Pentax came out with the 645 in the early 80's, wedding pros had already consolidated their equipment. Pentax was too late in the market to really compete for the wedding trade. Also, by that time, Pentax was "an amateur brand"--it was before the LX came out (which also came out too late to be competitive). Too many Uncle Harrys had a Spotmatic or K1000.
As much as I am deeply in love wtih the Pentax 67, I would never want to use one for a wedding. :) But for lots of other things, it was special. I recall a pro making photos of exhibition booths at the Offshore Technology Conference in Houston in about 1974. That was the first time I saw a Pentax 6x7 being used, and by a skilled pro. I fell in love from that moment, but couldn't afford to dip into it significantly until about a decade ago.
Truth to tell, I bought the 645NII because my wife was doing weddings with me, and she is no technician. She needed the automation provided by the NII--my C330 was utterly unusable by her. Then, the C330's film advance crapped out on the next gig, and fortunately I felt it happening and switched to my backup C33 for the rest of that wedding. I immediately bought a used N to go along with her NII. But the NII was still too much for her, so she went back to her Nikon for doing candids and I used the two 645's.
I seem to get pulled into doing weddings often enough to need all my past skills but not so often that they are easy to remember. I always have to do a little rehearsing with the camera beforehand.
Rick "old stories" Denney