I suppose it's more a matter of customer perception than anything else, because a small chip on the corner of the package has absolutely no effect on the operation of the much smaller silicon die in the middle; and if it's assembled into a product, the end-user is none the wiser.
Incidentally, modern high-density components look rather rough under the microscope too.
They arent worried about the chip. The fear is that the chip represents a mistake in handeling, was it dropped? You wouldnt buy new bulldozer with a dent even though nobody really cares. You need to know the dent isnt the result of something more important.
> Multiple chipped corners. Such damage often was a result of improper packing in an IC shipping tube.
If you're requiring mil-spec components, I think it's clear why such devices would be rejected. At the very least, the risk that improper handling has caused functional damage and/or structural weakness probably isn't worth taking.
Incidentally, modern high-density components look rather rough under the microscope too.