Regarding 2, is there any reason the union achievements must apply to non-union workers as well?
* Unions putting a lid on actual rank-and-file demands and initiatives.
* No or rare collective action.
* No solidarity action with non-unionized workers, workers from oppressed social groups etc.
* Political support by unions for the regime in general and for dominant factions within government in particular.
* Employers acting in concert with union by dues checkoff and requiring new hires to join (or go through the union to be employed).
* Legislation enshrining this arrangement, or preventing legal action against it.
... which explains why I didn't mention this consideration.
1. Union activities are fundamentally based on a perception of a common cause, common interests, common destiny of all workers - in your company, or in general.
2. It is a much weaker achievement to get an employer to give some people some more. The more meaningful struggle (and in fact - what is often the only real game you can play) is the struggle of "what is an employee doing some kind of work entitled to" - that is, a struggle over what's standard, what's necessary, what's appropriate vs unacceptable. But if there are two categories of rights/benefits, the better category is always that of privilege, and you've never convinced the employer nor yourselves that you are due any of those things by virtue of your being a worker. At most it is by virtue of some kind of alliance with the employer at the expense of the underprivileged.
3. The more fractured the modes of employment are, the more leeway managers and personnel dept. people have to manipulate and apply leverage to individual employees: "Will you be in status X or status Y next year? Hmm, let's see. Well, you did A, and we didn't really like that. I don't know if I can authorize you being a Y" and so on. Also, you (= the union) will be playing cat-and-mouse with the employer all the time about who gets employed in what status exactly; and the more combinations you get the harder it is to keep track of everything and also have a credible and actionable perception of what's it like for the various groups within the rank-and-file.
