I came across this rather old article on Usability from Jim Coplien (One of the guys behind Data Context Interaction). It is definitely worth reading, and here is a striking quote from it :
My points were that:
1. Test-driven development without architecture emphasizes a procedural architecture rather than the kind of good object-oriented architecture than supports the direct manipulation metaphor, which in turn is one foundation of a good user interface;
and 2. That the Agile Manifesto leaves usability at the side of the road
