And on the flip side, compensation plans that encourage people to stay to arbitrary dates are probably a mistake.

That's where I'm at today. There's a lot of money resting on my staying at a company for a year. I don't feel like I've been working effectively with the culture or team, and I'm pretty sure both me and the employer would be better served by my leaving, except the financial incentive, sunk cost, and avoiding having a "I worked here for only 6 months" on my resume is enough to justify me staying an extra 6 month.

Potentially not coincidentally "percent of engineers who stay a year" is one of the key metrics of the team that sets up the compensation structure...