I joined the IAM back in 1966 and have a withdrawal card dated in the early 1970s. Most of the meetings I attended were involved in inter-union arguments about what trades could bolt together what parts in industrial plants. Subsequently they expanded membership to airline baggage handlers..I kid you not! The full name is International Association of Machinists and Aerospace Workers. Find some guys who work in aircraft maintenance facilities and find out what their union has done for them.
To answer your question about good machinist joining unions; ever heard of closed shops? When it is join or no job offer, you join. And belonging to the IAM is no guarantee of ability; they never checked my papers, I do have them having served a full apprenticeship, they just took my word for it same as for all the others.