We too have many clients and many projects per client, all related to Web site or mobile app (a la iPhone/BlackBerry/etc.) development, and we see areas as being the main way to manage historical data related to profitability, efficiency, and long-term resource planning.
For instance, if over time I learn that 38% of a project's time is spent on the "QA" area or the "Design" area or the "Front-end Development" area (etc.), I can make smarter hiring/budgeting decisions for the long term.
Additionally, and again, this goes back to budgeting, we even have some clients that require we bill them by discipline, because we have a rate card with different hourly costs per discipline. While I could simply say that a CakePHP engineer costs $XX per hour, if that engineer is doing project management instead of CakePHP, their hourly rate will change. Again, with better use of Areas, this would be simple to track.
All that said, I would LOVE LOVE LOVE a way to have a "project template," which relates to a "global area list," because if we had project templates, or heck, even the ability to just clone a project's settings, we'd no longer have to go through the tedious task of entering in the areas each time. Although, even as I write this, I actually will go back to wanting global areas too, just for reporting purposes on an aggregate scale. But again, that's a nice to have. Project templates/project cloning is the key I think.