RFC 1939 describing the POP protocol askes for the mailbox to be locked after authentication of the user and before any access to the mailbox is granted. The mailbox stays locked until after the any changes are done to the mailbox after the client sends the QUIT command.
If mailboxes are saved in any of the formats where all the mails are saved in one big file, locking this file is actually necessary for insuring mailbox integrity. However, if mails are saved in one file per message (maildir format as POPular uses it), this is not strictly necessary.
If no locking is used the worst thing that can happen is that a user connects to a mailbox twice (or more times) and sees a message in the message list and when he tries to retrieve the message he will get an error message. While this is obviously not the best solution it isn't that bad.
On the other hand there is no reason why anyone will open several POP connections to the same mailbox. The only common case where this happens is when a POP connection breaks down on the client side, but the server hasn't noticed that yet. This can happen either because the client program or system crashes or the network connection (often going through a modem) fails.
In this situation not locking the mailbox will actually help. The first connection is inactive anyways and having a lock means that the server has to wait for a timeout before it unlocks the mailbox. Only after a timeout the mailbox is accessable again. Without locking the mailbox will be accessible immediately with no ill effects.
To make matters worse, indicating a 'mailbox locked' error to the user will often confuse the user and will cause him to call the providers helpline reporting a problem that has probably solved itself by the time the helpdesk knows about it.
For this reason there is no mailbox locking in pserv.