Proponents of unions seem not understand that unions only exist to enrich existing members and not benefit society, future members, the profession, or anything else.
There's a reason why new teachers, doing the same job with the same title, make less than half of what teachers with seniority do. There's a reason why many unions use credentialing as a way to restrict the free market and the only way new people can join is through nepotism.
Do unions proivde some benefits? Sure. Certainly the threat of a union can also provide said benefits. But unions, like all centralized power structures, eventually become corrupt and end up doing harm at the cost of protecting a small group.