[JoshTriplett]

That sounds like a really good argument for a union providing sufficiently valuable services to their members that they want to be a member. If a union isn't serving its members those members should be able to seek better options.

If unions worked like this in the US, it seems like many more people would be in favor of them.

[OkayPhysicist]

Unions by and large do work like this in the US. There's fewer unions, but the unions that do exist tend to do a very good job of representing their members. The main force acting against unionization efforts here isn't that the unions are ineffective, it's that they're so effective that companies fight tooth and nail to suppress them, including spreading a lot of propaganda.

And the problem with open shops and opt-in unions is that scabs can be found who think in terms of the short term. A company, in contrast, has the luxury of playing the long game. So, faced with union demands, a company can hire a bunch of scabs offering benefits and pay equivalent or better than the union is currently fighting for, then when the union is crushed gradually fire the scabs and replace them with new workers who are back to the square 1 the union was fighting from. And because unionizing takes effort and conviction, the new lot might not unionize. The company wins.