1. When you "as team leader" are not familiar with ASP.NET MVC.
2. When your project time is tight.
3. When you don't involved with the team from the beginning of the project.
4. When you want to introduce ORM like subsonic which is not mature yet (for ASP.NET MVC).
5. When your team is not familiar with MVC design pattern.
6. When your team is not familiar with (c# 3, moq, unit testing). 
7. When your team all consist of junior developers.

My opnion non mature team + Non mature technology = failure