Mirror lockup is used where any vibration at all might cause too much shake, such as when using a very long lens and slow shutter speed. Typically, first actuation of shutter release flips up the mirror, you wait for the vibrations to damp out, then second time trips the shutter. Eliminates vibration from mirror slapping up.
Not all DSLRs start at 200. The Canon 10D and 300D both have ISO 100. True, at ISO 200 there is very little visible noise, but there are times when lower sensitivity is needed, e.g. keeping shutter speed down in bright lighting.
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