The shutters in the consumer cameras are electronic based using bit-register shifting to control the light, and so are not mechanical. But, due to this, special microlenses over the CCD are required to focus the image. Plus, the consumer CCD's do not have the dynamic color range of the prosumer or professional SLR cameras.
So, the mechanical shutter and lack of microlenses allows higher image sharpness. And the greater bit depth of the CCD in your D100 allows a greater range of color to be captured. That is why you have the Adobe RGB (1998) mode, with its greater range of color than the sRGB mode. There is a really good explanation of all of this at this website:
The microlenses don't focus the light (at least not relative to making the image any sharper), they act as photomultipliers that capture and focus more photons on each sensor to make up for the short exposure time of the bit-register design. It's not image degradation that requires the microlenses, it's the need to put more photons onto each pixel.
That said, it's very possible to use a full-frame sensor for in a live composition mode. The pro studio backs pretty much all do that. You use a computer monitor as your "groundglass". But you'd have to shift between viewing and shooting with a shutter close cycle before you captured the image.