From my understanding, the real value of Areas of Focus in GTD is to help ensure you're balancing your projects; that is, you're not doing too much in certain areas and neglecting others.
With that understanding, I don't think the Areas implementation in Nirvana is the best.
I tag my projects with the Area to which they align. In doing so I guess I can review my projects by one Area at a time, but what I think is lacking is the ability to stand back and look at all areas to see imbalances.
What I have used it for is to assist with context. My computer context is too long, since I'm in front of a computer all day at work. When I filter by computer context I can then further filter by the, Procurement or Service Management area for example, and work on next actions in one area.