I'm not arguing for unionization here, although it does have its advantages; I'm arguing that:
1) Unionization is easier and more effective when defection is difficult (or impossible, as in some German unions) and heavily punished. As soon as defection pays off, it loses its value.
2) The set of high level executives at large corporations is small and cliquish enough that it is a de facto union, since they can convince boards to bid up each other's prices and punish/shun those who offer their services at lower prices.