When the eclass implicitly adds implementation flags to IUSE, the dependencies basd on them should use python_gen_cond_dep() function rather than be inlined. This guarantees that once the support for a particular implementation is removed, the ebuild isn't left with stray USE references. We would use a repoman check for that ;).
repoman support has been removed per bug 835013. Please file a new bug (or, I suppose, reopen this one) if you feel this check is still applicable to pkgcheck and doesn't already exist.
