I think I can answer this. First of all, the quality of images created by the upper-end (60-80MP) Hasselblad digital (Hasselblad camera + formerly Imacon digital backs) cameras, the Pentax and Mamiya cams, and PhaseOne produced backs with their open adaptability to many medium format cameras, is very, very good - superior to any 35mm sized sensor based camera. (DXO is smoking crack here as their rating system scores Nikony sensors ridiculously higher than most medium format digital because of the value placed on qualities not demanded by the market for medium format backs, like high ISO value performance). However, despite what I just said, the real question one should ask, with the Nikon D800E already here and the much rumored Canon 46 MP monster just around the corner, is just how much better are those MF images, and is the price differential worth it. For 97% of commercial shooters, and even for the vast majority of status seeking and well heeled photo-hobby-crazed orthodontists, the answer is probably a strong "no." The system lenses, ease of use, autofocus, environmental sealing, frames per second, shutter speed variation, accessories, auto exposure options, adequate to very good HD video output, and even the huge number of menu driven control options - not to mention image quality that is steadily creeping up to within a few histogram humps of reaching medium format territory - means never having to say you're sorry that you bought into a smaller format system.
When you can produce a great looking 3-page pullout in an offset magazine ad, create a standard-size poster, and inkjet output a great looking 20"x30" color print, all with a 35mm format digital camera (already do-able with the D800 Nikon for sure and, probably, with a Canon 5D3 as well), what real rationalization, except billboards and even bigger inkjet prints, is there for medium format? Well, there are at least two I can think of - one obvious, the other less so. The obvious one is much mentioned and, unfortunately, rings very true in my experience. It is that medium format, like a Hasselblad, is a very expensive status symbol, and the more expensive something is, the more status that can be attached to it. $40,000.00 and change buys a lot of status. And, that status can be used, if the model for this behavior holds, to impress the people who might want to spend great gobs of money on you taking some sort of pictures for them. In their minds, the more expensive are my photographer's tools, the better he (she) must be, and therefore, the more discerning that I, the client, must be. Plus, there is a plethora of new kinds of assistants whose jobs also depend on the obscure software that attaches to these less popular cameras - the digital assistants - who will swear and attest to the wonderfulness of these very expensive tools that, as the keepers of their esoteric flames, guarantee them some much needed income. The less obvious reason for a commercial shooter to own one of these medium format devices is a matter of convenience to their clients. When one has a very lucrative photo job to let out, the universe of potential photographers who can actually do the job well is rather large - too large for the AD's, Creative Directors, Art Buyers, and Picture Editors to comfortably wade through with their complex bids and explanations. If they really considered the available talent pool, no job could ever get awarded in a timely matter - too much time spent away from the more pleasurable aspects of their jobs and just plain too slow a process. So, as in all such circumstances (think admissions directors at Ivy League colleges and Universities using 4.0 gradepoints and stratospheric SAT scores to weed out the students who could matriculate, but can't possibly all fit in the freshman class), it's very much easier and faster for them to, consciously or not, simply weed out those whose gear doesn't "measure up." This is not always how it works, but, unfortunately, it is how it mostly works. So, again, if you're angling after some really big buck jobs, medium format digital is a potential aid to help you catch some of them, and possibly a neceesity for entre into the pool from which the actual job winners are chosen.
I completely agreed with dafrank, IMHO, it has pretty much similar analogy of Canon L lens vs 3rd Party Lens. Many 3rd Party lens have comparable optical or image quality as those "L" lens, and given 2 photos, not many can tell one is shoot by "L" and the 3rd party, and yet many (especially among wedding photographers) will prefer to carry a "L" for their assignments simply because of the "Red Band" status ... giving the client the cult-like "Professional" impression so that one can demand more!
... And it certainly applies to carrying a Medium Format around compare to a DSLR. To give everyone a real world comparison .... please check this out .... http://www.photigy.com/nikon-d800e-test-review-vs-hasselblad-h4d40-35mm-against-medium-format/
For the above review, the major difference I can see is the color shift or color difference between Hasselblad and Nikon. I strongly suspect, if the author of the review fully perform a color calibration to both sensors before the review, the difference won't be that obvious ... especially in the skin tone of the subject!