Some people don't feel that the union will change anything, or if it does, not for the better. A friend of mine is a member of the IBT and he's telling me constantly how the union keeps selling them up the river when a new contract comes out, and the scared sheep all vote yes because they're afraid of losing their jobs.
Further still, the guys at UPS Freight who I've spoken to (as well as Canada Cartage, their interline partner) have suggested that the union isn't doing much more for them than negotiating a contract. The pay gets better, the benefits are adjusted, but grievances are very often ignored. Many of the old timers there lament the loss of Overnite and the rise of the union.
I only know what I've heard, mind you. But there are many whom I've spoken to who feel that the union at least provides a voice where there would have been none. I personally prefer to speak for myself. And I will until the day I'm no longer heard.