扩展方法的用法 - 这不是一个糟糕的设计吗？

I have just started to look at .NET 3.5 so please forgive me if this type of question have been asked before. I am struggling with a decent usage for extension methods, in that I have just downloaded suteki shop an MVC ecommerce offering. In this project there is a pretty standard Repository pattern that extends IRepository. I have just started to look at .NET 3.5 so plea