REQUIRED_USE was primarily meant to avoid reverse-deps breakage, but this package is currently not used by anything else which makes the heavy use of REQUIRED_USE a moot point. It forces people to micro-manage USE flags which is not a usability improvement, but a necessary workaround in some situations. Inter-USE flag dependencies can be expressed in metadata.xml and warnings can be emitted in pkg_setup or pkg_postinst. If people don't read USE flag description, then that's their problem. This is not a hard rule and up to the maintainer.
Until migrating to REQUIRED_USE there was a HUGE mess in src_configure. I think current REQUIRED_USE is fine in mpv: it prevents compiling with wrong USE flags and makes enabling required features simpler. Also most of deps are already enabled in desktop profiles, so users do not need to change a lot USE flags.
