To be fair, train managers on many Intercity TOCs help with the catering service. On my own one they can often be found serving drinks or washing up at busy times.
If however they're suggesting running with fewer of the hosts required for a sleeping car service using the train manager in lieu of one or more of them, rather than just helping out with the general service when it's busy, then that I can understand.
The members however are often very heavily lead by the union. At my own branch of 200 odd members, about 10 usually bother to turn up to meetings, generally the ones with an agenda, and on their say so some pretty big decisions are taken.