Full disclosure: I have never been a member of a union, yet I am acutely aware that I have benefited from their existence or the threat of unionization. I am also aware that the essence of unionism is collective bargaining, so I make little distinction below.
Elementary: Are public unions necessary? Doesn't seem so. Just peg their salaries and benefits to the local private sector, unionized and non-unionized. Have public unions become too powerful? Seems so--and too expensive, relative to the private sector. Will elected and non-elected public officials ("management") take advantage of public employees without benefit of collective bargaining? Not likely, especially if their salaries and benefits are also pegged to the private sector. Can jobs between the public and private sector be compared. Nope, not much. Yet our military, which is not unionized and links its pay scales to the civilian sector and its pay raises to a cost of living adjustment appears to consistently achieve its recruiting and retention goals. Few argue that our military is overcompensated. Caveat: the military benefits somewhat from the efforts of various federal employee unions.
