with ~arch - I don't see the reason for a package.mask. Why not add some new tags like ~x86a for alphas, ~x86b for beta and ~x86 for testing as is...
because it's easier to block something in package.mask (edit one file instead of possibly a dozen ebuilds). Adding new tags isn't that easy as it requires code changes in portage and these changes have to be on everyones computer or they'll get errors about invalid KEYWORDS. And as you can undo maskings in package.mask with package.unmask in /etc/portage there is no reason to drop package.mask.
