Ah, I think you misunderstood me; I was approaching from the other angle. My point was: since all desktop OSes today give apps free reign - stuff like VAC is neccesary. Just like anti-malware, anti-rootkit, etc softwares - all of them neccesary in the given environment, even if it'd nice not having them running in the background 24/7.
Even Android - a relatively secure linux with some seperation between processes - will gladly allow you to mess around with any process, once you are root. So if you want to alter a process (inject cheat into a game, say)... then it won't stop you.
I understand the theory of having a properly compartmentalized OS (hypervisor running only completely sandboxed apps, etc), and I wouldn't mind having one. But, the thing is: we don't. Not in a state where they could run the majority of the apps we need an OS for today, anyway. Even so, we still want to play games. And without cheaters ruining it for all us, if possible.