There are some significant problems with locking while editing; your session lasts a few hours in the control panel, if one user opened an edit window for an entry and then walked away, did something else, or for whatever reason kept it open that long, no one else would be able to edit that entry for hours. That’s one reason that wikis, for instance, do not do that type of thing; a malicious user could effectively take over a site by vandalizing an entry and keeping his edits going, preventing anyone from fixing their modifications. Hopefully you do not have control panel access given to users who would do that type of thing, but even in normal usage, if your authors are not in the same physical room, or talking on the phone or via instant messaging, etc. then it could be very problematic to everyone’s workflow. And if they are already in the same room, on IM, etc., then they could just be communicating to others what they are working on to start with. And what about users who open many entries at once in browser tabs? They could lock out dozens of entries all at once.
If this happens often to your publishers, and they do not communicate with one another, what you can do is perhaps develop an extension that when a user opens an entry to edit it, it ‘flags’ the entry as being worked on. When they update the entry and save their changes, the flag could be cleared. When another user opens that entry, if the flag is set, it could display an alert that so-and-so was currently editing this entry and that your changes my overwrite or be overwritten by the other user.