Few things not listed here, like say, being on a "shared Network", it is not always possible for one to have a network to themselves, this, can be a vulnerability without any kind of detection/prevention.. Also, OS updates/program updates obviously can not be run in a sandbox either, while there is a very slight chance anything could happen, it is still a chance. I always recommend an av if if they are using sandboxie. To many variables to take a chance, and have them mad i said it would be cool to just use the sandbox, and watch them find a way to still get infected, as Umbra pointed out..