The semi silvered mirror would collect dust, and they're fairly fragile.
Thinking about it, dust on the semi-silvered mirror would be far less of a problem than dust on the sensor for the same reason that dusty lenses are rarely something to get worked up about - the dust isn't at the focal plane. Which makes me wonder if DSLR manufacturers couldn't all-but-eliminate the problem of sensor dust by putting in a piece of flat multi-coated glass (maybe even ED glass) between the mirror and the sensor's anti-aliasing filter. You could even make it as easy to change as a focussing screen. :-)
S
