I work in the Canadian federal context, so my perspective may be necessarily different than your own.
Federal workers (with the exception of managers) are largely represented by about 4-5 major unions, and a number of smaller ones representing specific job groups. You can find out more about that at the website of the National Joint Council, the national employer/bargaining-agent consultative body ( http://www.njc-cnm.gc.ca/index.php ).
In our federal system, much as in yours and systems everywhere, there is wage compression, such that the folks in lower level jobs make more than they would in the private sector, and the folks closer to the top make less than they would in the private sector. (Though 2 weeks ago, I was joking to colleagues that Mubarak hadn't really left, but was coming back as a consultant, because that's where the real money is."). Certainly one of the reasons why that is true is because blue collar and admin-support workers are generally un-represented (often by virtue of existing in small chunks) and short-changed in the private sector, while upper-level and management jobs are under-compensated because government has no means of generating revenue to pay higher salaries, and the public probably wouldn't stand for it even if they could.
Unions, much like firefighters, are entities that, when you need them, you really, really need them. In between those times when you need them, they like to keep their fire engine clean (It's a clean machine). Were the employer not quite so combattative, things could be more productive.
In past, I've had the pleasure to work closely with union reps when devising employee surveys and conducting analysis, and I have to say that there are a number of important issues that, if the unions weren't on top of it, those areas would be summarily ignored by the employer.
For example, just exactly who is tasked with monitoring the level of physical violence the nurses attending to vetereans face each and every day? If the union weren't on top of it, nobody else would be. Recently, our border guards were asked to carry sidearms, and I think there was even talk of parks rangers being asked to carry sidearms. Unions stepped in and said "Whoa, camel! I said WHOOOOOAAAAAAA camel!!". Indeed, when public servants get sh*t upon by public officials (and there is absolutely no indication that would not continue to occur, regardless of how many concessions they make), the only voice to stand up for them, as a body, IS the union. The employer may engage in forecasting labour needs, but often it is the unions who have a better sense of their membership, and can speak to the needs of the specific programs and services.
To whit, the Quebec provincial government just passed back-to-work legislation yesterday ( http://www.cbc.ca/news/canada/montreal/story/2011/02/22/quebec-lawy... ), to force provincial prosecutors back to work. The prosecutors, meanwhile, are quick to point out that unless the wages are made competitive with other provinces, their ranks will remain thin, and important tasks like investigation and prosecution of the assorted biker gangs and other organized crime groups that have plagued the province for decades, will ultimately crumble. Government looks at this and says "Well, we can't afford that pay increase". The union looks at it and says "Well you also can't afford to throw less money at it and have that money amount to absolutely no public good, either. So decide what you want.".
And that, I think, is one of the reasons why unions need to be there for the good of the government. Legislators rarely have any insight about things on the shop floor. They think in terms of abstract policy matters, and responding in ways that garner public trust. Those things ARE important, but then so is what happens on the shop floor.
Wisconsin's move to restrict bargaining agents to largely wage negotiations presumes a level of prescience about the concrete details of what permits programs to provide full value for citizens that legislators and senior manager share precious little track record of showing themselves.