Google's spiders index pages by following links from page to page across the web. If there are no links to the page, Google won't find it, nor will any other bots, including spam bots. Putting a reference in a robots file will alert spam bots to the presence of the page, which you don't want at all.
Of course, if you have a link on your site to a signup page etc., then yes, the page will be listed in search engines. So I'm really talking about a hidden login page for clients to log in to their CMS, for example.
Even with a login page that is listed in Google, it won't rank well and is unlikely to be found often in Google.