Digital backs use full-frame CCD technology, as opposed to Interline CCD technology, which is the one used in P&S and video cameras
The Dalsa 33MP sensor, for example, can provide a maximum of 4fps (RAW 16-bit stills) if all 4 outputs are used. Trouble is that the noise/signal ratio will go up and also there are no MF bodies that can take 4fps*
In Live View, Leaf backs do 5-6fps at 100% zoom as we utilise sub-sampling (something like binning)
CMOS, in a way, is closer to Interline CCDs. There's a Kodak 29MP Interline sensor that is used for industrial and aerial applications, however is is only 24x36mm....
So right now, if you want/ need to go either larger than 24x36 or more than 36MP, your answer is a digital back, proof.
* The AFi/Hy6 camera can do 4fps in mirror-up mode. I can show you a video that demonstrates it