But there's no evidence yet that it's being used by first-party programs, e.g. by GarageBand or Pages or Mail.app.

It's also quite likely that it's a) not being used at all, and the private API is just for internal testing until it's ready to be made public, and/or b) used by certain OS components that aren't competing with third-party apps (e.g. somewhere in the Settings panel).

And while I agree with your assertion in theory, some cosmetic styling is probably about the least important, most trivial example you could come up with... can't really get myself worked up about this one.