They should have stayed rare, but they’re a way for universities to get free money from the government as well as a temporary (but sadly permanent) fix for academia’s deep structural problems.
63 years ago, they were rare, but my father was able have his professor and department secure funding within a few months. ( yes, Engineering has a lot of money available from the defense department ).
50 years later, the Molecular Biology postdocs were not so lucky. Not even an honorarium. "Maybe in a few years..."
Just to raise the number from 0, I did exactly that. The post-doc was in a research institute, a department that focused on usability. It seemed worthwhile to learn how to really do user tests and usability evaluations in practice, in the best way possible academically (or so I thought) but with real enterprise projects. Contrasted a lot with the quite theoretical PhD.
We can always argue about whether it was worthwhile ofc. Moneywise, definitely not (though the pay wasn't all that bad compared to industry compensation in my countries). But I learned what I wanted to learn, and the issues the position had were speficic to the place, not the concept of the position, if that makes sense.
For the most part, it’s just another artificial rung the system uses to justify exploiting people.