Mirror induced camera shake can be easily visualized by taping, or rubber banding, a laser pointer to the camera and shining the light on a distant wall (12 feet or so away). Even with the camera on a weighted tripod you will see the laser beam's jitter on the wall when the shutter is fired.
I completely agree that mirror slap can significantly degrade image quality, and also that this is more apparent with digital than film. However, the laser test doesn't in itself conclusively demonstrate the effect, as the vibration could be caused by the mirror returning; or the laser's wobble may not actually coincide with the actual period of exposure. Furthermore, the vibration may well derive from the shutter rather than the mirror (and focal plane shutters are generally worse than leaf shuuters in this respect), even the friction within a mechanical cable release can impart some degree of vibration into the exposure.
Zeiss's Photographic Lens Marketing Manager once posted an interesting claim that most home grown lens tests designed to explore image quality have limited validity, because unless an impractically heavy tripod is used there'll be some degradation of quality due to vibration even with the mirror locked up.