Easier said then done. Firstly it requires for you to know what you’re doing is both a key project and something in which paying more would be more likely to result in its completion.
Is it though? I've never worked as an actual employee, but I'm working in long-time contracts as a freelancer with companies. I've always found it pretty easy to tell who was carrying a project and who was dead weight. Might be different and harder to calculate for new projects, but for existing ones that are maintained, observing who gets asked when weird things happen and who solves the strange issues usually identified the people who were instrumental. If someone is off a week and nothing moves, that's someone you probably want to keep (and probably also a situation you want to resolve, because that's not a good thing).
Managers responsible for promoting usually aren't in the trenches and getting honest feedback is actually hard.
When asking people about the performance of their peers, you usually get equally positive feedback unless folks are fucking up.
You have a good point though. Perhaps managers shouldn't be asking for feedback on specific individuals, but ask individuals who they go to for help and mentoring.