Very clearly all those are serious advantages with MFDB, the S2 is an MF DSLR. With an MFDB you can switch bodies and you can even use film as backup. Of course, if you only carry a single zoom lens to Namiba and the front lens falls out, nothing of that will help ;-(
It is very clear that a lot of flexibility is gained with MF Digital Backs. One real advantage with MF is resolution/sharpness. If an MF back has a small pitch it will have many pixels, has it large pitch it will stress the lens less. Potentially a larger sensor would also collect more photons and therefore have less noise. So MFDBs have a real advantage in image quality.
With the Leica S2 much of the flexibility is given up, opting for a smaller and possibly more efficient package. There is no reason at all an MFDB owner would switch to a DSLR if the advantages of that system don't fit his/hers needs. That said it may be a bad advise to suggest that MFDBs are needed for professional work. Both systems have strengths and weaknesses.
There are many issues with the Leica S2, to begin with it is very expensive, even if it is competitive with some MFDBs. But, those MFDBs don't exist in vacuum. Phase and probably have decent upgrade policies, Leica can have that to, but before offering upgrades they need to survive (no small matter in this times).
In my view the Leica S2 is probably very competitive regarding optics, perhaps less so on sensors. Phase and Hassy have long experience with digital backs and had all opportunities to make all the mistakes it takes to gain knowledge, Leica is a beginner in this area.
Finally I'm not sure that MFDBs are the real competitors to the Leica S2, but both the Nikon D3X (and it's lesser colleagues) and the new Pentax 645D may be.
Did I even mention DR ( BTW I would never say 6 stops EVER and I have never commented on DR and that comparison between 35 and MF anywhere on the internet ) or it is blowing a D3X out of the water, too the contrary the DR on the S2 is less than the Phase P40+ by almost 3/4 of a stop or more, real world with testing. Not me and that is Rays quote not mine.
As far as the D3x I have no idea and never mentioned a word about it since I have never tested it. I honestly don't care about a D3x and what it can do or not. It's not MF and that is what I shoot. Let's be careful who you are addressing your slams too.
But the flexibility and inconvenience of MF is a misleading comment on Rays part. Lets break it down if you want. Let's take a P40+ back for example . I currently have 4 Mamiya bodies dating back to at least 1990 or longer that I can put it on and I have anything that Phase develops this year , next year or maybe even 5 years from now to put on a body without switching a mount. Number two I can switch to a Contax mount, okay that is a dead system but I can. Hassy H1 and H2 I can switch the back mount too. On too lenses depending on system I have everything Mamiya has ever made and what Phase labels today, I also have all Contax Zeiss lenses, than all Hassy H lenses and if I want I can bolt on any V Hassy lens.
Next I have ANY tech camera I want to bolt onto. Alpa , Arca, Horseman, Cambo, Silvestri and also almost any view camera style cam i can bolt that back too plus let's not forget all the glass from Rodenstock and Schnieder.
That is a ton of flexibility and also I can retain my back for a lot of years and use on any of these combinations plus any future cam that comes along. You can't say that with a Canon 5dII, sure you can bolt some lenses on it outside Canon and has some other capabilities but a MF can do a lot more moving around. Not to say it won't cost you money but look at all that glass I can play with and all the bodies I can bolt this darn thing too. There is a lot of power in this that get's left behind in almost every conversation.
Now lets bring this back to the S2 it is even less flexible than a Canon or Nikon. It ONLY takes 3 lenses to date and you can't move the body around. Not a Pro in there right mind would even consider this for there only ONE system. Any Phase and Hassy shooter has a far better chance to call there system the only one they have because they have backup bodies they can use and a entire system like written above to work with. 35mm you have to buy another cam for backup which fortunately does not cost the price of a S2 and is far more affordable to do. But almost any 35mm cam today is considered disposable as not very many folks will even have the same body after 3 years. Now that is reality