Not necessary or feasible, as there are many apps that you'll never see on the Mac App Store, because the developer doesn't want to pay Apple a percentage. The solution is to exercise caution when choosing and installing software, no matter what source.
Authorizing an installation that you didn't deliberately initiate is not being careful or knowledgeable. MacDefender is 100% preventable with prudent user action.
I never said that wasn't the case. However, the Windows viruses that remain in the wild can infect a system without the user's knowledge or permission. Such a threat doesn't exist in the wild for Mac OS X.
Like every other OS, Mac OS has never been immune. Nothing has changed.
The only false sense of security is held by those who have selective hearing or reading and choose to ignore common sense. The only security current Mac OS X needs is a user being prudent in their actions.
That is not false, as no viruses exist in the wild for Mac OS X to "get".
... which is false, of course. The problem isn't the OS. The problem is, and always has been, the user being uninformed, misinformed or careless.