I use Antivirus Free. I scans installs and downloads and uses very little resources. As for the "need"? I have never found anything and I'm not sure there is anything to find. I have been watching this thread hoping for an answer too. I would say use Antivirus Free or another free one until someone actually confirms a need at all.
Paid antivirus is more powerful.I used to use free antivirus,however,it can't detect the virus exactly .Since I bought Norton 360 ,I never worried about virus or Trojans,or other threatens.