The implication in the comment I was replying to was that if people see the value in something, they will voluntarily contribute to its maintenance.

The fact that many workers would not pay optional dues does not prove that the workers don't see value in the union.

Obviously the purported value of a union is in the ability to collectively bargain on behalf of its members (and to provide tangible support for workers participating in strikes or who are pursuing legal action against an employer). Unions may do this or not, but the argument that gp made is not a good one.