Most practical SoCs are more than just a collection if IPs and custom development. There is a clear hierarchy associated with the IPs that are used.
Paul McLellan's blog "The Earth is Not Flat; Neither is IP" on SemiWiki presents a number of advantages to organizing your IP in a hierarchical manner. Read Paul's complete article here.
You can also download our white paper titled "Hierarchical IP Subsystems" and other interesting white papers on the Resources page of our website here: http://www.methodics.com/resource-library