If they have had access to a shared account, how much work are you willing to put in to verify that there's nothing in that shared account that will get executed by another user later that will quietly reinstate a key?

A reason to have separate accounts is that not only do you terminate access, you also have an easier time ensuring that less of what that person had access to could have been compromised. (This of course goes right out the window if said person has sudo/su access, in which case you have a much harder time, but even then giving them individual accounts means your opportunity to audit becomes so much greater)

After all, it's not the honest guy who'll never try to log in again you're primarily trying to protect against (in fact: for the honest people, a good security policy protects them by making them less likely to become potential suspects if/when something happens - it's in your own interest when you leave an organisation to ensure you get locked out), but the guy who might decide to try to do something later, or who might even be thinking about doing something before they leave.