It occurred to me that Magnus Eriksson wrote in gmane.os.netbsd.general:
>    And from now on, always use the "package" target when installing 
> something, it'll save you trouble later on.  If... sorry, *when* the 
> recommended tool of the week trashes your installed packages, you can at 
> least reinstall them via "bin-install" (or even pkg_add), without having 
> to recompile all of them again.

In that light, setting DEPENDS_TARGET to "package" in /etc/mk.conf helps.
'make package' will only package the current package. If any dependencies
need to be installed, they will not be packaged, AFAIK.

And yes, to top it off: using pkg_comp or any other chroot environment to
build your updated binary packages trims it off. But still, it's far from
perfect.