I do not think admin templates should used. But something very simple and dumb (that looks similar to browser HTTP auth window). Here is example:
http://pastebin.com/nnX36RB6Css should be moved to static files, of course, to make this template cleaner and slightly more customizable.
Imagine I need to create working solution as fast as I can. I really do not care about good UI now, because I need only a prototype to show it to my customer. I will not spent time creating nice login page: I just simply copy/paste template from Django documentation. And login page from contrib is great aid here. I will probably change login page UI later, but it is minor issue and priority is low. Many people in Intranet solutions (portals, CRMs and so on) are happy with browser HTTP auth window. They really do not care about how ugly is it.