I missed that pretty important distinction, actually. I'm glad you pointed that out.
I don't disagree that the bargaining costs don't always necessitate the dues. But workers are also paying for the networking of organizing and promoting certain movements, for example. As with any organization, the cost of operation is a bit more than the direct service provided.
One caveat that I'm familiar with is that of Teacher's unions. I come from a long line of educators, and there are certain things that happen at the State level, for example, that are mandated by legislators who have absolutely no idea what they're doing (to be curt.) I'm of the belief that professionals in any given field are the best voices for change that may (or may not) need to occur in that field. When non-professionals legislate certain methods or monetary restrictions on a broadly important part of society, some manner of organization needs to exist in order to protect that profession, if not only the professionals.