I think the answer here is compatibility. But we finally are in a turning point. OSX's new apps and windows metro apps are sandboxed.

But until running mostly apps becomes the norm in a desktop system beware that not having admin privileges doesn't not mean you can NOT: load programs at startup, read most of registry settings, passwords, read memory of/close programs of same sec level. A malware doesnt need admin rights to do evil.

Still I believe AV products are useless even for inexperienced users.