I don't use them, personally I think if you use some foresight into what you are installing then you don't need antivirus on Android.
Most malware attacks on Android are through apps, these apps can be on the Google Play store but are dealt with reasonably quickly by Google once reported. There have been malware apps in recent times that send text messages to a premium rate number owned by the people behind the app and this app can make your device hide from you the user that these text messages are being sent. Obviously this doesn't affect the N7 so not relevant but the best way to protect your device is to be aware of the permissions you grant applications, if an app is asking for permissions that you as a user doesn't think it needs don't install it.
Some of these malware apps are also able to root your device, what they can do once this is done is access user information including password etc.
A newly reported piece of malware for android is from the net, you open a web page and you are informed that you cannot access the page without installing an application and then your device will start downloading said application, obviously it doesn't install automatically so you are safe unless you the user install the app. Your device protects you to some extent from this because of the "unknown sources" option in settings, if you have ticked this to allow the sideloading if apps then make sure you cancel the download it refuse install.
It's up to you though, I personally don't think Android Antivirus applications are worth installing but you choose because it's your device and I'm telling you this so you the user can make an informed decision.