Granted it would take a LOT of spam to use 5GB a month but it is theoretically possible (yes I know everything works in theory and we should all move there
)
The other points are very valid though. Virus attacks which install backdoors etc. have been problems in the past. Even the probing from some of the viruses caused serious net problems and generated huge amounts of traffic. Again the probing would not be enough to cause cap overage in it's own right but at the same time the traffic would be attributed to a users monthly total. The users will still be picking up the tab for unsolicited traffic.