Management by metric
Management by metric is generally doomed to failure, because you end up rewarding people who are good at gaming the system, and getting rid of people who are crap at this sort of thing, but rather good at their specialisation. Essentially, management are trying to use this as a technological 'fix' to avoid actually having to do their jobs as a managers - which is to know your team.
What should be happening is that managers should be completing the skills inventory for each of their team members, and each completed inventory should then be reviewed by the team member - not the other way round. The manager's manager (and HR) then gets to see the result, and managers who produce inaccurate assessments are dealt with.
Self-assessment works well for people who are good at trumpeting their skills. It's not so good for the less outgoing people who quietly get on with the job.
Getting subordinates to complete self-assessments just gives managers another sick to beat people with: have you completed your self-assessment yet? Any problems with it are your fault/responsibility etc etc.
Of course managers don't want to complete the assessments themselves: its a lot of work when done properly, and subject to tiresome grievance processes when you get it wrong. It's entirely understandable they want to shift the responsibility downwards. And wrong.