There is that, yes. But when it comes to actually PMing a member or looking up someone's information via the ACP/MCP for whatever reason, if a member had recently changed his or her username, the staff will have a hard time tracking down that particular member if they do not know what username they are going by...unless of course they start to click on random members' profiles via Statistics or Who's Been Online list. And if that member happens to be not available in that list, then I suppose we could start asking around in the SB and see if anyone knows where in the world Carmen is if we're that desperate.
On a more serious note, I still do run into that problem even with the name restrictions. Giving everyone the freedom to change their usernames whenever they can (on a daily basis) would be out of the question.