In theory right to work is a good idea, it makes unions abide to free market principles and if workers don't want unions the unions wont be there.
However any homo economus (A profit-motif rational species used frequently in economics models but is rarely found in the real world) would not pay a union fee.
If the union will fight for me regardless of the free, why should I pay? In the long run unions will die with RTW legislation. Good riddance why have them? If they can't survive in a free market, then they're not needed.
However thats not entirely true, unions negotiate with employers to make sure they give employees fair wages for their labour. I am against RTW, even though I want weaker unions, having them there make sense, and if unions go, the people will regret it.
Sometimes the government knows something the common man doesn't. Most workers have never taken buisness courses, and have no idea how to properly manage finances, they'll just see the union fee, see that its optional, and will gladly rid themselves of it. A union negotiates, and both the union backed workers and the non-union backed workers get a salary increase, workers paying the free will see that the unions will back them regardless of if they pay. So what do they do, they stop paying. Soon the unions are bankrupt and go, so what happens, their salaries are cut and they can no longer protest. They will regret doing this in the long run, but not in the short run.
Unions are one of those things that are needed by can't survive in the free market.