A number of packages have fortran interfaces, in particular some I am making ebuild files for. If a fortran use flag were available, compilation time/bloat could be reduced for those who don't need fortran, and better support for those who do. Ideally, the gcc ebuild would use the flag when its being configured, so g77 would be built if set. If there's some process for requesting USE flags, LMK.
do enough users care about having/not having fortran interfaces? to be honest, even though I love debloating, I have never been concerned about the fortran thing