Oh baloney. This would only "prevent" install-and-authenticate malware, and if malware for OS X continues to evolve as it has on Windows, you may not need to actually run an installer. Anyone who turns it off won't get any kind of warning and malware continues. Those who leave it on probably won't get anything outside of the App Store, which is the real idea behind this: Scare the newbies into giving more money to Apple.
UNIX is designed so that you can't have viruses, only malware that relies on the user entering his/her password and allowing it to install. They just need to have it warn you once WITHOUT A POPUP that it is unsigned (instead of a dialog box every single time like in Windows).
"App seems legit, and I don't need Gatekeeper? Hmmm, it warned me that it is unsigned. Now it's asking me for the password. Better check on this."