The problem is that informal policies can be bastardized by individual managers. Relying on rapport and good will doesn't work with narcissists (an extreme case

The problem is that informal policies can be bastardized by individual managers. Relying on rapport and good will doesn't work with narcissists (an extreme case) or people with different values (more mundane), for instance. And different values are common in most workplaces-- almost everyone has a slightly different answer for how much leave is too much.