Link state protocols and Areas concept
Link state protocols have introduced the concept of multiple routing areas withing the same routing domain. Link state protocols depend on the fact that all routers must have an identical link state database and then each router will start calculating its very own routing table from this information.
However, this rule sometimes introduce scalability limitations to network designers. In very large networks all routers must maintain the same link state database; this induces some scalability limitations in these networks.
One of these limitations is that all routers in the network must have sufficient processing power and storage capacity to handle the large link state database. Another kind of limitation is that database flooding and synchronization operations may consume lots of the available network resources causing spikes in both bandwidth and processor utilization.
The concept of Areas comes in hand to resolve these kind of scalability limitations. By dividing the large IGP domain into multiple areas we can isolate network changes, link state database flooding/sync operations within the single area. Routers with low memory and processing power can still be used by putting them in small areas
But what about real life situations ?! From my own experience I have not seen networks applying this concept of Areas to solve scalability issues. I have worked for two service providers one of them is running OSPF in the core and the other is running IS-IS and both of them have no scalability problems with a single area design, specially with the increasing of processing and memory power of today's routers.
Multiple areas can also be used to control traffic from one part of your network to another even if not having scalability limitations.
End of story is that areas are one tool in your arsenal as a network designer, you can still use in your network whenever you find a real need for using them, however the main rule is to keep things simple.
That was my experince with area desing, what do you think from your own experince?