Trial employment is a great way to not be able to hire the best. A trial period means uncertainty. Any reasonable candidate will consider that a negative, compared to a competing offer which looks more certain.
More specifically, this is another way for an employer to move the risk of hiring back onto the employee. It's the employee who is going to need to leave their current position for an 'audition'. This is going to put the employee at a disadvantage during negotiations because the employee will likely not have as strong a position compared to have an existing job to fall back on.
So unless the 'best' are already unemployed, then this helps the employer reduce their risk, and improve their bargaining power during offer negotiations.
As a thought experiment, if you were trying to hire one of the 'best', during negotiations would you prefer if that person was currently employed or currently unemployed? As an employee, I'm glad that generally my BATNA includes staying at my current position (at a minimum).
I agree that interviews are broken; no matter what styles of interviews I've seen, I see them fail to predict the future.
Becoming good at firing is one way to triumph despite interview problems. Sometimes the employee is quite aware that they aren't working out. Even if they aren't, their coworkers probably can see the disaster unfolding. Done well, there's no morale hit.
