Since Patch X, admins can see other admins using the connection list and pressing shift-control as admins appear in a different colour.
Some people drive differently when they know there is an admin on the server. It was my understanding that it was felt that there shouldn't be an easy way for non-admins to know that there isn't an admin present so they don't abuse that fact. By not knowing they may find to their cost that one is present should they start doing things they shouldn't.
If I understand it correctly, the new conedodgers server system, when ready, will enable people to be given limited admin rights on the fly like you suggest, but I don't know how it achieves it.
Maxim