While I would love to have a p45+, even if I could afford it, I would still need a 1Ds mark 3. While the mark 3 can do it all and still produce great results even in a magazine spread (even when used in high iso), the backs have limited use out in the field. Wide angle coverage is limited compared with a dslr (i shoot a lot of architecture) and high iso capability from what ive seen is strictly limited to iso 400 and only in emergency situations. In the film production sets that I sometimes shoot in there isnt a lot of light and even with high speed glass (which MF doesnt really have) high iso is required.
Also, Moire, with the 1Ds 3, no problem. With some phase backs that ive seen, it can become an issue that verges from minor inconvenience to a nightamare situation.
"Centerfold" issue with MFDB's, havent seen it first hand but read of a few cases. yikes.
Speed and reliability, 1Ds3, no contest. The backs while sturdy (phase seems to be the most reliable) arent sealed to the elements, battery performance is mediocre at best and the cameras dont like the heat and humidity (i live in the tropics)
I recently worked with Chris G (cool guy,
http://www.westsidestudio.com ) from Westside studio and he was using a 645AFD2 with p45+ and the awesome 28mm mamiya lens (lens alone costs almost as much as a mark3). The files looked great but he was very weary of using iso 400 in one shot. It was an outdoor shot but light was low and people were part of the shot and motion blur was a concern.
So is the 1Ds mark3, "jack of all trades, master of none"?, well, its the best DSLR ever made, I have no doubt about this. We might be a tad spoiled nowadays with all the great choices in DSLRs that a camera like the 1Ds mark 3 might not look that impressive but man when I finally got it and used it for my work. Wow. Just like the 5D is the best compact DSLR ever made.
I still remember my first DSLR a fuji s2. I used it for several years and actually kept it until recently. I took 2 shots with both cameras (5D and fuji) of the same subject and composition, one at iso 200 and another at iso 800. Processed the raw images and took a look in photoshop (side by side). The 5D image just blew away the fuji S2 images even though i have made some awesome prints with the older camera and shot quite a few ad campaigns with it. 100% in ps is just brutal with gear.