There is a business case: if, for instance, Linux is left in the dust at web serving because Apache uses memcpy significantly and Linux's memcpy is three or four times slower on a P4 than the version used by W2K.
If apache is block copying 32K chunks around then there is a much better cure - fixing apache. It isnt "lets do big memory copies fast" first of all but "why am I doing big memory copies".
In the case of the PIV I've also seen little evidence to suggest any meaningful different between rep movs and fancy stuff even on big blocks unless you have innate knowledge of cache handling and local use (which glibc doesn't). Intel seem to have nice silicon fast paths for this stuff