The better answer is the enemy has changed. In the past, security focused on multiple users of the same machine. Users should be safeguarded from each other.

The better answer is the enemy has changed. In the past, security focused on multiple users of the same machine. Users should be safeguarded from each other. Applications are installed system wide by a knowledgeable sysadmin and can be trusted. But this threat model was completely dated. Today, most machines have 1 user. But the applications are either cheaply written with no respect for other applications, or are actively harming the user by stealing his data, and the average user is his own sysadmin but really doesn't want to do the job or knows how. Additionally, DRM means the machine tries to defend itself from its owner, preferring multinationals instead. The windows XP and standard unix security model defend against the old threat model. Android, iPhone and Vista + defend against the new model.