Yes, whether it is a username or a real name, that would be private data which is up to the individual to disclose or not. I can see a use case for it, but if this information is available to other subscribers, which seems to be the case on most lists, I'm not sure if the use would be desirable.
With the new setup, such information would no longer be list specific, as the account is wikimedia-wide.
What you seem to be looking for is a freeform field for the user-list combination that is only accessible to admins (presumably on the 'member options' page). This could contain a username (assuming you got permission for that during the registration process), but also other information such as expiry date or something, or whether someone has been warned for misbehavior. I could see the upside for such a generic field, but I'm not sure whether it is easy to implement.
I note that there is already a 'username' field in the user settings that the user can choose to enter, but I'm not sure where this is exposed, if anywhere.
Lodewijk