Pretty straightforward; if you're going to use package.use.mask, either your stuck doing the usual 6 month wait, or you need the appropriate portage version dep in packages. Lack it, no gurantee you've got a portage version supporting package.use.mask...
*BSD is still experimental and unsupported. It has enough fixes going into portage and other packages for obscure issues that the latest version of every system package is almost always required; updating the profile every time a new release comes out is pointless. We do have a portage version supporting package.use.mask, because we always require the latest release.
