With optional union membership, you get into the free rider problem, unless you think the union should be able to negotiate a limit on the wages of non-members as well, so that the cost of union membership is compensated.
I'm not sure what you mean by "special privileges". I'm in favor of practicality.
I understand the concept of the "free rider problem", but I'm not sure it would manifest itself as much if everyone was left to their own devices to negotiate the terms of employment. If a company had half their workforce under a union, and half not, the half not a part of the union get zero of the potential benefits of those negotiations (growing wages, more time off, etc - whatever terms the unionized members wanted to negotiate). There are very few topics of negotiation between unions and employers that would be almost impossible to segregate between the unionized and not-unionized (safety conditions is the only one I can think of off the top of my head).
There may be employees that want different terms of employment than the union is looking for. Maybe they only want to work 25 hours a week, or they want to work 60. Maybe they would prefer to buy their own health insurance. Maybe they want more pay than the union is able to negotiate on their behalf.
Just my "free-will" individualist perspective.
I'm no union law expert, but the special privileges I'm referring to is just any government intervention on how those union negotiations must happen, or how the relationship is arranged. If the union wants to take their labor elsewhere (to a competitor), they can, if the employer wants to end the relationship, they can.
(Sorry, I didn't mean to hijack this from the original topic)