When you run an application that needs administrative rights (in this specific case via a manifest file), you are prompted with an UAC dialog to allow this operation:
This is the dialog you get for the "default" user, the one you create during setup that is a member of the Administrators group. Contrast that to the dialog a standard user is presented with:
Now, I am fine with prompting the user to enter administrative credentials. However, I am not fine with providing the user with the name of the administrative user(s) on that machine. In my opinion, this is giving away security-related information without need.
Update Of course you can always use net localgroup Administrators to get a list of the members of the Administrators group (Markus pinged me on that one). This feature has been available for ages, true. However, I am not convinced that the UAC convenience of providing the administrative accounts on a silver platter is really necessary.