Once when I was refactoring my code, I went up the IDE to the using section of my C# class, and cleaned unused namespaces and duplicate namespaces, and sorted them all.
My pair (pair programming) asked me the reason. I had no idea why I did that. I did it out of my habit to keep all of my code clean and tidy. I mean, I told him that having a cleaner code is a good idea in general, but of course that reason was not a good justification, as I won't even bother to spend my time in the using section of any C# code page.
Since many times you move a class or an enum (or a type in general) from one namespace to another namespace, and this adds new using statements to your code (either manually by going up the code window and writing the using statement yourself, or through the editor using Alt+Ctrl+F10 combination), and since these new using statements would be added to the end of using section, which makes them not sorted alphabetically, and since compiler never complaints about any of these problems, why should we care to make this section clean and tidy? What reasons might we have?