Neither the Leica nor the Phase are big, heavy, cumbersome or slow. I used the Phase 645DF with the 80mm lens, the 45mm lens and the 75-150mm zoom lens for the whole week we were there. In fact, in terms of dimensions the Phase is a flatter camera than my Canon 1Ds Mk3. It is a bit heavier, but not by much with the single focal length lenses. With the 75-150 it's got to be somewhat heavier than a Canon 1Ds3 with a 24-105 zoom just because there is more glass, but I was surpised by how manageable it is. I was expecting much worse. Also the form factor is pretty good, so it's no more cumbersome than our larger DSLRs. I would have been happier with one or two of the buttons placed a bit differently, but that's a quibble. As for responsiveness, it really is good. The autofocus on this model in particular is VERY responsive. It takes longer to process images because there is so much more data to process, but here too, much faster than I was expecting (using Sandisk Extreme 4 cards) . As I said, this isn't gear for sports photography, but your characterization of it is truly way off the mark. If you can borrow access to one of these specific models and haven't done so yet, use one for half a day and you'll see what I mean. Likewise for the image quality.
I was involved in a blind-tasting of rum back in the mid 1960s when I was teaching in the Caribbean for a 2 year stint (done to raise funds for good causes). We designed the experiment very scientifically and the results were really interesting. The rums which people normally considered beneath their dignity (too cheap, "common stuff" in the grocery store) turned out to be the favoured brands under blind-tasting. So I know exactly where you're coming from. And it is true that the overall quality of digital imaging has reached a stage where it can be difficult to discern what kind of camera produced what images; but all that said, there are just enough instances I've seen and experienced where it seems like MF, smells like MF, looks like MF and is MF. I won't tire of my 1DsMk3, but as I said, each has its place in the sun and when MF is used to its fullest comparative advantage there is no mistaking what it is.
Big, heavy, cumbersome and slow are relative terms. I admit the Leica S2 is an example of an MF digital camera which is not excessively big and heavy compared with a 1Ds3 or D3x (if you discount the lenses), contrary to the usual situation with MFDB, but it's greatly lacking in many of the prized features sported by cutting-edge 35mm DSLRs.
Consider just one factor of cutting edge technology, lens Image Stabilisation. This is cutting edge technology which is far more significant (in my view) than some slight increase in resolution due to an increased pixel count. Absolute lens quality is a significant issue for me, but technology which allows one to realise the maximum potential of any lens in practical situations is surely more significant. I bought a new camera recently, the D700, mainly on the impressive qualities of just one lens, the Nikkor 14-24/2.8. It's the only Nikkor lens I own. I can't find another one that interests me. If the recent Canon 100/2.8 IS Macro were a Nikkor lens, I'd buy it in a flash. The fact that the Nikkor 14-24 does not have VR is not such an issue because it's such a wide-angle lens which doesn't require a particularly fast shutter speed for a sharp result.
Cutting-edge technology is the 8 fps of the Canon 7D at an affordable price. The Leica S2 has double the pixel count of the 7D but 1/5th of the frame rate. How cutting edge is that? 15x the price but almost 3x as slow. Not impressed!
I appreciate the fact that, without stitching, an MFDB like the P65+ is the only solution for impressive results at huge print sizes. But this is a very limited application, ie. subjects which do not lend themselves to the stitching process because of significant subject movement. I understand that it's not practical to get a model to hold perfectly still, without blinking, so one can take a series of shots of various portions of her face and then stitch them together to make a gigantic, high resolution poster. That would be very restrictive. If you are in the business of producing gigantic, high resolution posters that bear scrutiny from up-close, then a P65+ may be justified.
My printer is the relatively small Epson 7600. The wonderful flexibility of cutting-edge 35mm format DSLRs is far more significant and exciting for me.