Project areas carry an administrative overhead. Access security is an obvious reason for having multiple project areas.
What do you see as the reasons for having multiple project areas in an enterprise?

The decision to use multiple project areas is based on your scope. Do you have different teams with different needs? Different plans? Different guidelines for different projects? Different processes that distinct teams follow? If everyone does everything exactly the same way then it a single project area works perfectly. As the number of distinctions in how things are handled on this project versus that project increases, the nicety of multiple projects comes clearer.

Having said that, the upcoming 4.0 release will highlight Process Customization and the ability to use Team Area support to isolate teams within a single project.