... Most people who are already in a current job are not going to be willing to have open debates about the job opportunities they are seeking. Not only because their 'boss' will know but also because if they are in a public company that could cause large issues in the market etc....
Do we really want to select against people who aren't open to discussion of their individual merits, even if that means selecting against those who already have a job they want to keep?
This is additional support for the use of a (potentially mutual) Co-director position, which would be an effective way to have a trial period for any candidate. It doesn't have to be high-stakes up front, and would be easy to revert in the case of bad fit or Board regret.