I'm with you, Sam - you make some excellent points, most of which get overlooked as the gear-heads strive for the biggest and best. And I speak as one of them - I'm probably more guilty than most when it comes to chasing the 'best' equipment. My stereo is unsuitable for my house, my car is undrivable in Hong Kong, etc...
So when the P45 came out, I tested it for several days and was very excited about it. No doubt, it was fantastic - loved it, and still want one. But when it came to the decision to part with so much cash, even I had to question whether it was worth it for the uses I would get from it. Unfortunately the answer was, and still is no. I printed shots up to 24"x36" to compare with my 1DsII and I could see the P45's were 'better'. But, none of my friends could say which was best - they could see they were a little different (didn't get colour balancing and compositions exactly the same between cameras), but not which was best - even at 3 feet high. Back it went to the dealer.
Eventually, I still want a digital back I can use on my old and much-loved Mamiya RZ67 house brick, but not at these prices when my Canon gives me such good results and versatility.