If the WM does not understand those hints, they'll be simply not displayed. If a Plasma theme has not been developed with that in mind, it can look awkward at times but it'll still work!
KWin could remove all non-GL code right now and KPD would still work with OpenBox, TWM, etc.
The most retarded party in that area is Canonical. They already had a Unity version (Unity2D) that was not a WM plugin. They could simply have said: “Look, we develop Unity[2D] with Compiz in mind. If Compiz doesn’t work, it'll fall back to another WM. We don’t support this, we won’t write special fixes if Unity under Metacity will look strange at time, etc. It is just meant as a stopgap until you can install proper drivers.”
But no, they decided to throw away the superior technology to concentrate on the plugin-based version.