I see your point.
On the candidate side, as a PM, I’ve always asked for this and have had the chance with startups and one big company. It wasn't a formal contract, rather just poping up in the office for a week or two.
Main reason has been: PM can be a great experience in some companies and a living hell at others. I personally use this to audit myself if the environment would allow me to achieve the goals I’m commiting to. My interviewers knew how hard it is to find the right PM, so I was clearly signaling how much I cared about joining, which becomes a levarage for me in the process.
On the company side, when recruiting, we have always looked for candidates who really want to work with us. Being a startup, we considered finding someone who personally cares about what we are building as key. So having someone in tells a lot about the candidates interest and as you say, reduces our risk of hiring.
I can really see, though, how this has a completly opposite vision to it. As you say, this one is tricky!
Thanks for the reply!