maybe this is a naive perspective, but it seems to me that the union should have more to offer labor than simply middlemanning access to an employer. if people genuinely want to be in the union, it should be hard to find non-union workers.
The trouble with completely optional membership is that it introduces the good ol' Free Rider Problem, bane of many an attempt to make things better without forcing anyone to do anything.
It is. Everyone wants to work for union shops like UPS or John Deere, and people feel it's a great victory to be hired. While non-union shops are always hiring, because they drive people into the ground.