[chousuke]

The difference is in the kind of audit trail it leaves. If a sysadmin impersonates a user, that leaves a different kind of trail than a user logging in with their own key that only they can access.

In principle, the sysadmin should explicitly avoid knowing any of the user's secrets, because if they do, the user can shift blame onto them: "It wasn't me, it was the admin!".

I will never generate a private key for a user and will initiate a reset process for any passwords and other personal secrets revealed to me; to do anything else would be irresponsible.