The unions in slovenia opose the reduction of income tax for workers (after years of saying that they want to tax the capital more and work less). So yeah... unions in some places are politics first, their own pockets above that, and workers fifth.
This very same argument could be made against liberal democracies, where there is a potential for similar dynamics of corruption to develop. However, almost no one would argue that liberal democracy isn't preferable to a system where one person or a small group can decide everything.
Unions give you a vote and that is better than the alternative. Preventing a union from becoming corrupted is another problem and there is a wide spectrum of structures among unions, not all are as you depict.
If most of the unions were good, and just a few corrupt, people would look differently at them... When the situation is reversed, with many corrupt ones, and only a few not, people stop supporting them.
