There are many applications that have a need to store billions of entity-attribute-value type records, in addition to their more traditional relational data. An example of this would be Microsoft's own Entity Framework product. I think it's in Microsoft's best interest to demonstrate that, for this scenario, the best solution is for a relational database (like SQL Server) to co-exist side by side with a key-value optimized database solution. Attempts at implementing large scale key-value data models in SQL Server will result in frustrated customers.

Essentially it's same concept as moving OLAP cube data outside SQL Server and into a seperate database product, like Microsoft Analysis Services or moving BLOBs into FileStream.