I've been an early adopter of this since May 2012. I've used it quite a lot and it's great. The mileage is going to be on how other courier services are accepted. If you have a package couriered to this address from anyone but StarTrack, it's not going to be accepted. So for national deliveries this service is great, however international shipping (which is generally couriered) ends up being a problem. Either way you should sign up if you're Australian, this is the best thing Auspost have done in years.
Well root is the default full access account. It is a bit easier to root@serveraddress and brute force in that way than needing to guess which account in the system has sudo access.
It also becomes easier to detect when someone is attempting to break in when you can see logs of common user names in a row fail to log in.
I don't do much admin outside my hobby boxes. One guess would be, excluding the ability to enter a root shell, each sudo command is logged so you know who ran it. If the user is root, then it's harder to know what they did while they were root.
Disabling root also means you now have to guess the username and the password instead of just the password.