I've had avast installed for a couple years. It's got a great security/location suite, which is why I installed it. I have NEVER had any sort of virus warning. I've never run a scan, since that's not why I installed it, but the lack of any sort of warnings over the course of 2ish years tells me the antivirus is pretty unnecessary.
FWIW, I've downloaded a handful of potentially dangerous files. I don't get involved in app piracy, but I've downloaded a handful of MP3's, ebooks, etc.