If it's truly not a commodity market, then a firm could gain a competitive advantage by refusing to negotiate with the union and hiring non-union employees.
That amounts to the same thing.
This retains the problem that you have to join the union, and therefore are immediately subject to the union's negotiated rules on seniority, pay scale, benefits, etc. It's better than a closed shop, but doesn't solve the problem at hand here.
My employer doesn't do yearly bargaining with the union - in part, probably, because employees haven't demanded it. The last time the unions had major discussions was when our employer wanted us to sign new contracts, and the unions negotiated to make sure we weren't going to be prevented from doing things outside of work. Generally speaking, the only reason union members bring in the union these days is if they're being terminated, and the union helps make sure it was done properly and works to get the worker the best package possible.
Conversely, if you're older, or have health issues, those group plans can be a steal of a deal.