When you hire someone or form a partnership, you would be wise to set some milestones to shoot for.
But I've seen a lot of people misuse milestones. You should use milestones to reward. You shouldn't use milestones to punish.
Organizations that value linchpins seem to be focused on this reward aspect of meeting milestones.
Organizations with a lot of turnover and no room for linchpins punish people for falling short of milestones.