> The gist of it is that employees aren't compelled to pay union dues...
This is always the argument unions make to get mandatory contributions from employees. The risks of doing so are the following:
1) that all employees regardless of their individual contribution get the same salaries, meaning that over-achieving workers will work less, and under-achieving workers will not work more. (a.k.a. elimination of incentives)
2) That employees that do not like how the union is managed cannot work in that place, either because the union would not allow them, or because they have to pay for an organization that they do not belong to voluntarily
3) That the union, having sole negotiation power for all workers, takes kickbacks from the company for lower compensation for all workers.
4) That even if the union is incorruptible, its optional but its subscribed by most employees, that it actually manages not to decrease productivity, then, there is another company that does not have the same problem, will attract more capital since it gets bigger returns, and the company fails anyway.
Unions are not economically efficient organizations. They are politically efficient, so they serve a great political purpose, but their work in the economic sphere is only destructive.