http://host.com/?form
- works for the whole site

http://host.com/path/?form
- works only for /path/

How common is to have the same login form in multiple places?
 - Lwn.net in its articles (/lwn/article/####) has the same form, you would have to put your login every time for every article
 - Google accounts?
 - Twitter (and other RoR sites I believe) has:
  + /login/
  + /
  + /sessions/new/

Path based logins allow you to have a user in /admin/ and another one in /login/. This is a *workaround* for fixing multiple users per site.