Is there a reason to use a instead of a:link or a:visited in my stylesheet?

Can anyone tell me if there's any reason to use a {} instead of (or adjacent to) a:link, a:visited {} in my stylesheet? I've had inconsistent results testing this across browsers, sites, etc, so I've never come up with a satisfactory answer. I was not able to find an answer on The Google, either.

This is of course assuming there would never be a reason to use an <a> tag without an href value. Maybe that's a mistaken assumption.

** EDIT **

I know what these selectors do. I'm looking for reasons why I would use a instead of a:link or a:visited, since a always has an href attribute. But as Chris Blake and Ryan P said in the answers, there is also <a name="top"> and javascript usage, neither of which would need the href attribute.