What should the "Show Recents" menu item do?
The official documentation is very thin for messages (it doesn't even have an entry in the Applications section of OS X support online)

It could be working correctly if it's just a stub to allow you to see some recent messages when the application is not running, but that functionality isn't present today. Once you start up the app, the Show Recents menu item in the dock is suppressed and you instead get to show the various windows that the app could have open.

So - it's useless and could get hidden if it's simply a bug or perhaps an update will add this functionality so that it actually shows something that has happened recently since it doesn't show any recent activity at 10.8 build 12A269.