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To piggyback on what's been said. Android tells you everytime it downloads or installs something, so you don't need to worry about having malware installed without realizing it like on a PC.
The big risk is when you install apps that aren't from trusted markets (the Play Store and the Amazon App Store are the big ones). Install something that not from the play store is called "sideloading". There's an option that says "don't allow installation from unknown sources" that controls whether or not you can sideload or not. By default you can't. The only issue is even to install something from a trusted market like the Amazon App Store is considered sideloading and you have to disable that security setting. But it still informs you whenever it's installing something so you still don't have anything to worry about.
That being said, many of the security programs have other good features besides just anti-malware. For example I use Lookout, not because I'm worried about Malware but because they offer lost device location services using your phone's GPS. You can sound a siren on a lost or stolen phone. And you can even remotely wipe a stolen phone. IMO, those services are much more valuable than the virus scanning.