In my view/experience labour movements have generally pursued job stability as a primary instrument (for worker rights), even if it was not formally a goal in itself. This may be somewhat at odds with solidarity, but every movement deals with some internal contradiction.

There a multiple reasons for the focus on permanent employment. One is the labour union history of labour movements. Unions represent members and their interests, which are protecting their jobs, not jobs in general. Another reason is practical: it's almost impossible to unionize and hard to apply protections to a casual workforce. Labour movements (esp unions) are empowered by permanent workforces. Another reason (possibly the dominant one) is simple practical politics: people want a guarantee of permanence.

I believe that casual employment types have a role, in many cases one that permanent employment struggles to fill. From the worker perspective, this has to do with the low commitment, low barriers to entry, and relative absence of gatekeepers. Historically, when labour movements have tried to regulate casual labour they have tried to turn it into permanent labour.

In my view this is misguided. They should try to insert worker rights into the class while maintaining the casual noncommittal aspects of it.

For a practical example, I recently talked to some squatter/anarchists. They're hardly a major group, but weird and marginal enough that they're good at illustrating a point. Many of them work in the Gig economy (bike delivery). No Gatekeepers is what makes it work for them.