No matter how I look at it, I can't see any net good for voters when public employees unionize. Public employees are meant to serve the community they are employed in. The benefits of taking a government job are supposed to be in that you have a direct result in helping the communities around you; instead, the benefits most people see in government jobs are fat pensions and a bureaucracy that, if you're clever enough, you can get lost enough in to where you don't actually need to work to get paid. If anything ever hits the fan, you have multimillion-dollar, politically-connected attorneys ready to serve you.
In the case of police unions, it strengthens the "thin blue line" into a powerful bureaucracy that only looks out for the interests of its members, very often to the detriment of the people these public servants are supposed to be serving (and always in cases such as we've seen in Chicagoland recently.)
I would love counterexamples to this thought. I can't seem to find any of recent noteworthiness, but that could be due to the current anti-law-enforcement/anti-government streak getting pageviews in the news these days.