When would you use Simple subdivision rather than Catmull-Clark subdivision? Subdivision is selected as Catmull-Clark by default. However, the Blender development team still includes the Simple subdivision in Blender 2.72. Does it serve a purpose? Why should I use the Simple subdivision instead of the Catmull-Clark subdivision?   

Here is a picture of the Subsurface panel. You can clearly see the options Catmull-Clark and Simple.
As an example of a case where simple is might be preferred over Catmull-Clark, say you have a large plane which you want to subdivide non-destructively for use with the displace modifier.

When subdivided with Catmull-Clark, it becomes nearly a circle:



But with Simple it maintains the original square shape:



This is just one example. There are probably many such use-cases where Catmull-Clark subdivision doesn't provide the desired result without extra things like edge-creases.