We all know that Scrum proposes us only 3 roles: Product Owner, ScrumMaster and Team Member. During my work as a trainer I saw many teams that failed to manage project architecture, technical decisions and design solutions in new more flexible way. Without strong technical background team members make mistakes. Those mistakes are very hard to detect and prevent without strong technical leadership and well established engineering process. There should be somebody responsible for both of these things and Tech Lead is a very suitable role for this purpose. I believe that Agile projects can easily live without Project Manager but missing Tech Lead role will cost team much more.