I'd assumed that if a root password was set, the GUI would ask also, and if there isn't one you don't get asked. However, I've always assumed that, so I could be very wrong.
Given the amount of power the GUI offers the user, the more things you can now do with v6, and the fact that unRAID is appealing to broader audiences (corporate archives, etc.), I would say that the option to hide the GUI behind a login is essential.
I understand that you need to get to it as quickly as possible (and a summary page, or the option to permanently skip the login, or a remember me option would solve these), but when somebody can start, stop, add and remove virtual machines, create, delete and modify network shares, unassign drives, power down, etc., it would seem scary to leave it wide open.
Anyways, I came here to work out what dual parity was, and I now know, I think, so thanks one and all. About £180.