However, giving that information right before a decision point (i.e. when changing jobs, getting a promotion, etc) is just inviting it to be misused, it's a risk they shouldn't be required or always expected to take.

For some context, some time ago I was involved in handling quite a few maternity/replacement situations. Our local laws are quite generous with maternity leave, so for existing employees we (in that company) had often started planning replacements as early as 6 months before start of their leave. However, this is a bit tricky - if there was some promotion issue in the middle of that period (3 months after I'd known but 3 months before she'd leave), I could try acting as if that information was unknown, but it's difficult, I can't magically unlearn things, I couldn't be sure myself if I wasn't biased one way or another (unless we had extremely dry formal procedure based on some arbitrary "objective" metrics that mismeasure as much as they measure), and I definitely couldn't prove that I wasn't influenced if that was contested. So in such an situation I might have even preferred not to know, despite the extra hassle it would cause.

It's probably a question of trust. If an employee is working in a trustworthy environment, they'd likely volunteer that information (which I've seen happen) and the employer would benefit from having created such an environment. But if employees feel a lack of trust, they would likely want to protect themselves and keep it a secret (which I've also seen happen), and IMHO they have (or should have) a right to do so if they really feel the need. The employer's reputation and trustworthiness is their own fault or achievement based on their previous actions, so in some sense they deserve what they get, whether it's trust or lack of it.