It is not unusual to have a user who needs to impersonate another user without having that user's password.
- a secretary may need to REGISTER to receive calls for somebody
- a named user (e.g. email@example.com) may need to make a call using some role identity (e.g. From: firstname.lastname@example.org)
The CommonNameMappings feature for TLS partially addresses this requirement. However, a more unified approach, not specific to TLS, may be better.