Virtual1 mentioned the non-blank password requirement for sudo commands. This may sound far from your bed, but it isn't really. Among other things listed in this thread, leaving open your account password affects your ability to fix Home folder permissions. Many users sooner or later run into problems that require them to fix Home folder permissions (also known as ACLs or Access Control Lists), or live with the limitations the issue imposes, like the inability to change the name of files and folders, or to move or delete them. ACLs are not affected when you repair (System) permissions with Disk Utility etc. See for an example the Apple KB article Mac OS X v10.5: Renaming or... folder (note that this issue is not specific to Mac OS X 10.5).
