the bindist USE flag is only used by 3 ebuilds: xfree-4.3.0-r2 freetype-2.1.4 freetype-2.1.4-r1 IMO that's a reason to make it a local USE flag. Also the name and the description for it is a bit irritating as it has nothing to do with GRP or binary packages, it currently only affects truetype fonts in xfree and bytecode interpreter in freetype.
actually no, it does have to do with GRP. The purpose of the flag is GRP, in fact.