I hope you're not defending the practise? It shouldn't matter how effective the practise was, it's still wrong.
I especially don't mind it if, as the great-grandparent suggests, the existence of other non-cartel competitors meant it had no effect on salaries.
Even if it did have indirect effects on salary levels, such conventions might also plausibly help prevent certain kinds of ruinous/negative-sum-for-all competition, that rewards a few rapid job-switchers, but at the expense of the industry, region, or majority of employees.
I would mind a "don't-hire-if-they-apply" agreement. That'd be an abusive oligopsony conspiracy in restraint of employees' ability to seek out new arrangements.