Why need to purge user manually?

Yesterday, I mentioned that a Delete action via Delegated Admin will not remove/purge a user permanently in Sun Messaging Server. Read here. 

This is where Sun Messaging Server differ from the other email products. A delete action does not actually remove/purge a user's mailbox, nor does it remove the user from the Directory Server. It merely set the inetUserStatus flag to "deleted".

I can provide one of the reasons why the product is built as such.

Sun Messaging Server is built with ISP in mind. As such, there must be a way for billing ($$$).

Can you see the "Billing ID" above?

For proper billing at the end of each month, all users (active or deleted) must be reflected correctly in the Sun Directory Server. Deleted Users should be purged only after this information is captured (usually via a Billing Application).