I think I can give a decent analogy for this.
A car can be driven by a competent person without a seat belt on, however a seatbelt is provided because not all drivers are competent, and you may have someone crash into you. Wearing a seatbelt is life-saving in this instance.
A computer can be used by a competent person without a virus scanner, or without a decent permissions system. However, a virus scanner and permissions system should be provided, because other users are not competent, and could mess up your settings with a virus.
It should be Microsoft's responsibility to provide a permissions system that doesn't allow viruses (or anything else) to harm a competent user.
A car can be driven by a competent person without a seat belt on, however a seatbelt is provided because not all drivers are competent, and you may have someone crash into you. Wearing a seatbelt is life-saving in this instance.
A computer can be used by a competent person without a virus scanner, or without a decent permissions system. However, a virus scanner and permissions system should be provided, because other users are not competent, and could mess up your settings with a virus.
It should be Microsoft's responsibility to provide a permissions system that doesn't allow viruses (or anything else) to harm a competent user.
