I wouldn't think that this is how you would hire a known performer. I can still see the problem, but I can also see how it wouldn't be a problem if it were a more common and more standardized process. And if the applicant were being paid contractor's wages (higher) during the tryout period, not salary-level wages.
Otherwise, that practice being more common means people can leave their job for weeks here and there no questions asked ("I'm going to work for our main competitor for 3 weeks" won't fly if it needs approval), and come back without impact or penalties on their performance.
That would be ideal, but we're so so far from that. Even getting people to properly take their parental leave is still a fight.
why not?