First off.. limp.. What the hell are you talking about? Anonymous access, at least on a UNIX machine will NOT devulge a passwd file. ftproot is setup to have it's own directory structure so that anonymous users cannot traverse outside of the daemon's root unless a user is logged in witha valid username and password.

..next..

What do you mean since it's multithreaded that you can leave document root? Hello. That's lame. Just because it's multi threaded doesn't meant that it can leave root. Multithreaded means that it can handle more than one connection (or process) at the same time. This has nothing to do witha poorly written FTP server not doign bounds checking on an anonymous connection to see if it is leaving root.
And, if a vlid username and password is given, it is SUPPOSED to leave ftp root, as a matter of fact, it's supposed to dump you in your home directory. As least on most UNIX setups.