From:  JTB
416.6 In reply to 416.5 
Units are very useful when
1. someone exchanges files with someone with a different unit system.
2. Modeling something with parts of different size especially when plans are used (a part is drawn and measured in cm, another in mm.)
3. Modeling something with many parts of different size and using many files and then combining those files into one. So when I model a car, the main body is modeled in meters, the wheels in centimeters and the bolts in millimeters. When I want to combine those models into one you will get a veeery big bolt!

Of course there is always the scale command but it works only in certain cases. It doesn't work when the file has multiple objects because the distance between them is changed too.