Interesting. I still don't see the point. Unless the person leaves for a new job and somehow hinders or negatively impacts (sharing inside info etc) your workplace it shouldn't matter where they go.
My husband left his last job 1.5yrs ago and took 3 other people with him. When their new company signed the 4 of them on they signed saying they would cover all costs of legal fees and rulings since my husbands last workplace had something in there about not being able to take coworkers with you when you left. But, for the new company taking a team of 4 strong workers on is more valuable than the potential legal fees it could have cost if the old company chose to sue (they didn't). That makes sense to me, but to say my husband couldn't work in the same field of business for 4 years if he left is absurd.