Every other operator, including the OAOs, has eventually decided to opt into Priv. That should perhaps give a clue as to the value which it carries as a perk, compared to the reduction in revenue it costs (if any).
I’m told some freight operators (DBS for one) don’t, perhaps because they can’t offer a reciprocal perk for TOC staff travelling as passengers you mentioned above!
That makes sense I suppose, but then raises the question of why LU staff get NR privs, when TfL doesn’t offer LU privs for non-safeguarded TOC staff!