Currently at least one ebuild helper function (newinitd) silently ignores extra arguments, which leads to some files not being installed (see bug 398655). All helper functions should instead die if they are given unexpected arguments to avoid such issues.
There's already a QA Notice in the latest release: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=e20df85cf8579aaa9cf07d59efc453d5837a7598 I suspect that this issue is rare enough that we can safely make it die for all EAPIs.