Though many reasonable rebuttals have been mentioned, the one case in which I find this pattern indispensable is when creating mock-instances for a variety of c

Though many reasonable rebuttals have been mentioned, the one case in which I find this pattern indispensable is when creating mock-instances for a variety of classes that need to be interleaved amongst genuine instances (say, ORM objects, if you're unlucky enough to have those in your life) so that code operating on collections of the latter doesn't get littered with special cases for the former.