[gok]

> There’s no reason a single union has to represent all workers in a field, any more than a single company has to be the only employer in a field.

There is a reason for it, actually: unions are exempt from antitrust regulations. Combined with closed shop unions (where legal) it can be effectively impossible to form a second union.

[mikeash]

Those would be reasons a union can represent all workers in a field, not reasons it has to.

This seems particularly pertinent when discussing the possibility of starting a new union in a field that traditionally hasn't had any: you're not going to suddenly get all tech workers to join up. There will be plenty of space for competing unions, and plenty of space for non-union workers.