[dec0dedab0de]

is there any example of a union for a profession where you change your employer every 2-3 years?

I know a few construction workers who essentially work for the union. A contractor will get a job, and they'll contact the different unions and say they need people to get it done.

[fludlight]

The unions in construction also provide continuous training for their members, and good training to boot. I've had real estate developers in NYC tell me they won't do foundation work with non-union labor because it's so important to get it right the first time.

[Johnny555]

Though the drawback is that union workers tend only to want to work with union workers. So you can't go with the union workers to do your foundation, and use a non-union contractor to do other work.

An employer I used to work for found this out the hard way when word got out that they had hired a non-union carpenter to do some custom cabinetry on an office build out (he was an expert craftsman who was not cheap, so it wasn't done for cost savings).

Suddenly it got very hard to find workers to finish the electrical and plumbing work.

[gaius]

Though the drawback is that union workers tend only to want to work with union worker

That is the whole point... a worker's only leverage is to withhold labour. As an individual that is insignificant but en masse...