Actually for engineering purposes, we always use mm. So I guess its just a force of habit for the guy and besides these are actually small distances, so no need saying something like 0.3 cm when 3mm is easily understood
In graphic design I see mm used quite a lot all the time. When I input measures in InDesign I always use mm. Standard measures are expressed in cm (like 70x100 or 21x29,7), but mm are also commonly used.