Implementing UX/UI change is not easy when code is architected without front-end flexibility in mind. If the process for making design changes is cumbersome, it slows down product iteration. Architect your app so that design changes and adding new features can be as frictionless as possible.
(Slides for my talk at Fluent 2013)
http://fluentconf.com/fluent2013/public/schedule/detail/28025