The venerable James Newkirk has a nice post about VSTS integration.  It came specifically in response to the “no unit testing in base VS?!?” blogging thread.  The gist of it follows.

Visual Studio Team System provides an environment which enables a number of development tools to be used and the user can get a consistent experience from implementation to implementation.

This is a critical sentence.  I've gotten asked “why another source control system?” in the past, and the above is exactly the right answer - we're trying to build a system, an entire development stack, that gives the developer/tester/architect/etc. the tools they need for fluid and integrated software development.  Yes, the Hatteras source control system is enterprise-class, with support for atomic changesets, renames, branch/merge that are actually usable, etc.; but it's within the VSTS whole that we (and other components) shine.  A lot of the credit goes to our scenario-based development approach and its focus on actual usage by customers.  The integration of VSTS enable users of the system to focus fewer seconds and brain cycles on the mindless tasks.  Face it, there are enough things to fill the mental cycles of IT devel shop people :)