What if they decide that College Grad #385 is "better at your job" than you are, despite no real-world experience in the position? To take it to an extreme ... what if every public school in America fired 100% of their teachers every year and replaced them with people with no experience, just so they could run at the lowest possible staffing budget? I know I'm talking in hyperbole here, but it's definitely a slippery slope in both directions. It should be a compromise IMO ... if you can show through mutually-agreed-upon measures that you reach X standard of work, you should not be able to be replaced by a cheap new college grad who is projected at that same X standard of work just because it costs less. Your job shouldn't be guaranteed "no matter what" but it also shouldn't be a frenetic 24/7 race between you and 100 people who want to take your job.
While generally I don't know a lot about unions, I agree that the desirableness of unions varies greatly from profession to profession. For example, the musician's union (AGMA) that many of my friends participate in performs a very important function of ensuring a standard range of pay rates throughout the country, so that no musicians or concert houses are getting away with undercutting others by extreme margins. The AGMA contracts state nothing about hiring or firing of musicians - that is all based on the talent of the players/singers, so no one is getting promoted or demoted for unfair reasons. This works, of course, because musicians usually get paid on a performance-by-performance basis, and not on a yearly salary.
I saw this article this morning and thought it was very interesting. Hopefully they're dead soon. http://www.reuters.com/article/2011/12/29/us-usa-autos-south-idUSTRE7BS0E020111229
Unions are generally a good idea, but not always beneficial. As a libertarian socialist I see them as a good way to organize regular working people and as a possible force of democratization, at least if more people threw off their irrational fear of being organized and more people got involved in the decision making processes within them, and in general they play an important role in evening out the income levels in their respective industries and society in general; at least that's the case in most western countries (except the US), where the unions actually carry some weight against the employers. In societies with a high degree of income equality and low relative poverty (both attributable to the work of unions), to me it seems there's less crime, particularly theft and corruption, and less social problems (except for religious and long-going racial issues). Also, what a lot of people don't seem to get is that the demands of a union are not always 100% realistic, it's all a game where you set the bar real high, so that after some strikes and negotiations, they at least get something instead of nothing. However, some unions (at least in countries where there are few other serious issues to deal with) seem to be run with the mindset that there's always more resources, more money, which obviously is not always the case, and seem to only be there to negotiate pay raises and keep inefficient (i.e. unprofitable) businesses alive. The latter in particular annoys me, but, on the other hand, there are profitable businesses being closed all the time, just for not being profitable enough for the owners, whatever that is supposed to mean.
A true capitalist loves unions and would step on his coworker's head to join one. Not sure why socialists are into unions, since all the unions I know are about gettin' that fat payday and not about redistributing the wealth. That's why I oppose unions.