I know of two area contractors who say "I used to belong, but the only thing they were good for was cashing my dues check." Another says "never again...the teachers in the apprentice program stole my best guys."
Which brings up one major advantage of such an association: the training program. To me, it is the height of short-sightedness to expect people to come to you, fully qualified, without your contributing to training.
Also, as Bill said, such groups often have insurance pools for various benefits. These benefits- be they health, retirement, tool rental, whatever- are often out of the reach of the small shop.