It also doesn't work for people who want to interview speculatively. Looking at what else is around once in a while is a pretty decent due-diligence that you can apply to your career. You can even be honest about this with recruiters. "I'm not sure that I want to change jobs right now, but I'm open to persuasion."

If trial weeks become commonplace, it starts looking like a form of structural lock-in.

This is why I heavily push to talk to nearly everyone on teams at the prospective company that I might be a good fit for. I spend 3-4 hours talking to various people over the course of 3-4 weeks. If the company doesn't allow me to talk to their employees, then I pass them up.

"unemployed" != "desperate"

Not for the first few weeks, usually. But given enough months the desperation is really just a quirk of the implementation.