The actual "necessity" of antivirus (AV) apps is a debatable subject, however I will agree that since there's little harm in running them, the "Better safe than sorry" approach is probably the best advisable route to take, especially since many of these apps, like AVG, also provide other anti-theft features as well.
Having said that, it's always important to keep in mind what AV apps are designed to do...*detect* malicious code. AV is only as good as the definitions and other detection methods they employ, and the developers of malicious apps will naturally always be one step ahead of those aiming to thwart them. It should also be noted that detection does not always equal removal, especially if the malicious app(s) are able to be installed or run prior to detection. In other words, one should not allow AV apps to provide a false sense of security to a degree where safe practices and common sense should be ignored. Whether you choose to run AV software on your phone is up to you, but should always follow the prescribed guidelines of never installing apps from unknown or un-trusted sources, and reviewing app permissions prior to installation. By simply doing this, you may not ever actually *need* AV software, but running AV software certainly doesn't make your device immune from attack either.
I make the above statement as a generally warning about AV software for any platform, not just Android phones, but also your home PC and other computing devices as well. AV software should be part of your defense against malicious software, but certainly not the only thing you rely on.