We first introduced basic build automation capabilities in TFS 2005 and, over the years, it has evolved substantially.  In TFS 2010, we introduced a major update which we now call “XAML build” because the build orchestration layer was based on the XAML workflow engine.  As that got adopted, we found that workflow was just not the best representation of build processes – wrong granularity for build, not natural to extend for build, provides capabilities (like durability) that really don’t apply to build, etc.