As has been noted, MFDB's are primarily marketed to and for professionals. Given the low cost of entry to the pro market these days (read: basic DSLR system), we are really talking mostly about the high-end of the pro market.
It may be hard to comprehend from the perspective of an amateur or hobbyist, but the decision to purchase a MFDB, or to upgrade an existing one is not a mind-boggling prospect as some seem to think.
I can't speak for all shooters, but I know many who share my thinking. A decision to spend say USD50k on a system, which sounds like a large wedge of silver, has to be viewed in context of its earning potential over its expected service life (for me, that is typically 3-5years), the tax benefits over that time of depreciation, and its trade-in value at the end. If you are leasing, then you might consider the monthly cost versus the earning capacity. I expect my IQ180 to earn me a little over a million clams over the next 5 years, so would I lose too much sleep about the purchase? Not really.
Let me put it another way - would I be willing to allocate the fees from 12.5 days worth of shooting to cover this purchase, knowing that once I do, it will be 5 years before I have to do it again? If I was upgrading a back, this number is much smaller still.
Fortunately for the 3 players in the MFGB game, there are enough high-end shooters out there to keep them in business....