OK, I'll throw some gasoline on the fire :-)
I shoot primarily landscapes and use a P45+ on a Mamiya 645 II, as well as a D3 (one of these days I'll upgrade to a D3X). The only thing the P45+ gives me is more pixels, and no AA filter getting in the way of those pixels. There, I said it out loud. Believe me, I take no pleasure in saying that, given how much I spent on the P45+ kit. But that's what I'm seeing.
More pixels is no small thing, and the lack of an AA filter further helps out, and in fact these are the two primary reasons I got the P45+ kit. The detail available in a MFDB file is breathtaking. But as for all the other so-called advantages attributed to MFDBs, I'm not seeing them. If anything, I agree with DxO's assessment here. Maybe these other advantages were real a few years ago, but with the latest crop of DSLRs, the gap has narrowed considerably. I have no trouble pulling clean sharp detail out of the shadows with my D3. If anything, I rank the D3 slightly superior to the P45+ in this regard (based on using the D3 at ISO 200 and the P45+ at ISO 50, their respective base ISOs). And the D3 has more mid-tone to highlight DR than the P45+, again at base ISOs. The P45+ saturates at around 2 and 2/3 to 3 stops over mid-tone. The D3 highlights don't hit the limit for another two thirds to a full stop beyond that. And the D3X is reportedly even better in this regard.
On top of all this is the convenience factor in Nikon's (and Canon's) favor. I have excellent quality super wides to super telephotos, and everything in between. I have an excellent selection of tilt/shift lenses. I have blazingly fast AF. My batteries last days, not hours. And I have superb in-camera live view that allows me to achieve critical focus to a much more repeatable and precise degree than I've ever been able to achieve with my MF kit.
One of these days I'll get a D3X, and I doubt I'll pull the P45+ kit out much after that, based on what I've seen to date.