To expound on the security "issue" with default shares....
The only way one could add a connection to a share on an NT-derived Windows OS without having to authenticate is if the local security token (username + password) matches an account in the remote SAM database. In other words, if you're logged in as administrator with no password and you "net use \\remote\c$" and machine "remote" also has an administrator account with no password, it will be successful; else, not.
It's an entirely different game with Windows OS' that do not have SAM databases (i.e. they have those horrible pwl files in the %windir%).
Additionally, the only way to add a connection to a remote box for which you don't have a valid account is through a dictionary attack and/or brute force. I've written command line tools for both methods, and I'd be happy to explain the method for those who are curious. There are other available tools such as nat that perform similar functionality.