Well, it is in Administrators now.
Personally, I think that robust WebDAV access to Briefcase (including RW) would be great. So if you think you've found a bug, you should report it.
However, it works fine under MacOS X 10.6/ZCS 6.0.12 NE. I just tested. I put a few items into the Briefcase of a test account, shared the briefcase to myself, and accessed the following URL:
https://zimbra.company.com/dav/test@company.com
Also works fine with Windows XP (configured as a Network Place), although double-clicking on a file tries to open it via my web browser, which then tries to re-authenticate. However I can copy the file to my desktop, or delete it in situ, without a problem.
With Windows 7, I could authenticate, but the DAV folder was empty. The thread you linked has some extra steps, but I didn't try them.
Looking further, it seems that WebDAV in Windows 7 had a bunch of problems out of the gate. Not sure if they've been fixed. Some Google searching on generic issues with Win7 & WebDAV turns up a couple threads with suggestions:
can not connect to WEBDAV server ? Webdav
You could also try another WebDAV client, e.g.:
FTP, SFTP and WebDAV client