TINAS- It's the SL bit of the SLR, photogs want to see what their camera will see through its lens, not through a tiny low res screen on the back. Without the shutter and mirror setup you need a second lens for the viewfinder and you wont see the same results as the sensor sees.
In DSLRs the shutter is a rolling physical shutter, the bit that makes all the noise is the mirror moving out of the way. It's the shutter that fails normally, but its not too expensive to replace. Some use electronic shutters in "live view" mode, which has its place.
Part of the problem is that when the sensor is exposed it heats up, creating noise. the larger the sensor and lens assembly, the more heat produced. If you were to use an elec shutter only, you'd find that the sensor would need more coolinga nd would be more likely to produce noise (AFAIK).
