Short sprints keep focus on delivering value. This is true but such as any project management framework a great focus os put on short term results. Who is going to care about design on the long term. The solution to this is to add Scott Ambler DAD role, the Architecture Owner, have a look at:
http://disciplinedagiledelivery.wordpress.com/2012/05/28/the-dad-role-of-architecture-owner/