The only reason to try to test interoperability of implementations using
private use values is because the thing they are doing with private use
values cannot be done in a standard fashion and that thing is important
enough that multi-vendor interoperability is important.
But if it is that important then why don't we come up with a standard way
to do it? Politics is one. Bureaucracy and WG inertia is another (that
it takes longer for the WG to decide something than for huge companies
to go through two complete product cycles is sad).
If a solution to a problem that people are demanding a solution to is
either politically forbidden or only likely to to come out in 3-5 years
then vendors are going to bypass the process.
Taking away private use values because people are misusing them (how to
standardize something outside of the standardization process) will only
cause the workarounds they devise to be more novel and problematic. We
should fix the problem that is causing them to misuse the private use
values in the first place.