Seriously, I don't know why this feature exists or who thought it would be a good idea. The time it takes to compile even the most complicated C# file is trivial.
On the other hand, when I try to access the .Take() method in System.Linq, or the Encoding class in System.Text, or suddenly I need a new List<T> from System.Collections.Generic - if you have used a tool to remove unused namespaces from your C# code file - you have made me wonder why my editor isn't detecting it.
There's a reason that many default namespaces are included with a default C# code file. Leave them there! Maintenance programmers will love you forever.