>Part of the reason for public employee unions is to protect the members specifically from political interference. The alternative is not necessarily "merit" based compensation but rather political favoritism and retribution.
And yet all forms of public sector employment, regardless of union status, are treated as staffing agencies who's hiring can be manipulated by those who traditionally hold the power of political interference. The best way to step up your career as a teacher, cop or other bureaucrat is to know a guy who knows a guy who's owed a favor by a politician who can write a recommendation on your behalf to an open position that you want to step up to. This is how people move from line level positions to administrative positions. (And before anyone says "but the police", they are somewhat insulated because they have strict traditions in their industry that have sway over career advancement.)
If the purpose of unions is to insulate the labor pool from political meddling then they have done an incredibly poor job at it.
I'm much more inclined to believe that the purpose of unions is to extract maximum concessions from the employer(s) for the benefit of the people they control while ignoring any externalities. In settings where labor is interchangeable and employed privately the benefits are clear and the downsides are very limited. But when you start talking about the police and teachers unions circling the wagons to protect people who behave badly while simultaneously attempting to extract maximum money from society it becomes much less clear whether the unions in question are an overall good thing. It's one thing for the union to try and extract more concessions from a corporation that would otherwise pocket the money and supposedly has competition to keep them from just passing on the cost without pressure to reduce margins. It's a whole different ethical ballgame when society will be footing the bill and there is no competition to keep downward pressure on costs.