I get not accepting a counter-offer, although I know one employee that took a counter-offer that was insanely good and has been in golden handcuffs ever since, but couldn't be doing better financially.

However, I think that retention agreements and other "bonuses for staying on" can be helpful, even though I think that really what employees want typically are positive changes that would keep them from leaving (like firing a terrible manager or changing a policy) that cost less than retention agreements or counter-offers in many cases.

The best case is never to get in this situation by paying really well, hiring really well, and firing bad employees when necessary.