Whether every computer running Vista has multiple users or not, it is still a multi-user OS and all folder structure is designed around that concept. Even when there's only a single person using Vista it is still necessary to work within the multi-user paradigm in order for general computer usage to be performed with limited user privileges - helping to limit the damage done to the computer by viruses and other malware.
Yes, that's about the only 'benefit' for a user with an own pc, the potential help from potential viruses from potential people that click on/execute everything they see.
For someone that doesn't, the case is.. different.
