Does a MF back really have the 12 stop dynamic range you refer too? I find that amazing (hard to believe) as I have observed that all other digital sensors (from 2 MP digicams to a 1Ds) don't vary much at all and are substantially less than 12. But I do stand to be educated, having never seen let alone use one.
In actual practice, a lot of dynamic range will only produce incredibly flat images unless you really understand how to control local contrast well. I'm not saying it is not useful, but that most people would have no idea how to use it, or would lack the will to do the extensive editing necessary for each image to use it effectively. I would certainly not want by default to be using that range for every exposure!
Any more dynamic range on consumer cameras would just create photos that the typical user would be often disappointed in. I often find my students (when they first come to me) already complain about the "grey film" that seems to afflict a lot of their shots - a direct consequence of a flat tonal curve combined with a lot of dynamic range. It is easy to correct, and delivers a better photo than if the range wasn't there, but it requires custom editing to make it work, and few photographers with moderate experience understand this. They are amazed at how a simple curve corrections (often applied in local areas only) makes these images snap. But if you try to apply that type of curve in the camera to every shot you would end up with terrible results in different circumstances.
What would be needed to make more dynamic range useful for general use would be automatic curve corrections based on scene contrast and a data base (much like the process of evaluative metering). I'm sure it will come someday, and when it does it will, like most automatic controls, improve the quality of snapshots, but not quite deliver the results obtained manually.