Why would I write a fake class and unit test it?

I have actually found two uses for the mock classes that we use in repository implementation testing.

The first is to test the services that use an implementation of the "ISomethingRepository" equivalent that you mention. However, our repository implementations are created by a factory. This means that we do write tests against the "ISomethingRepository", but not against the "MockSomethingRepository" directly. By testing against the interface, we can easily assert that the code coverage for our tests cover 100% of the interface. Code reviews provide simple verification that new interface members are tested. Even if the developers are running against the mock that the factory returns, the build server has a different configuration that tests against the concrete implementation that the factory returns within the nightly builds. It provides the best of both worlds, in terms of test coverage and local performance.

The second use is one that I am surprised that no one else has mentioned. My team is responsible for the middle tier. Our web developers are responsible for the front end of the web products. By building out mock repository implementations, there is not the artificial obstacle of waiting for the database to be modeled and implemented prior to the front-end work starting. Views can be written that will be built off of the mock to provide a minimal amount of "real" data to meet the expectations of the web developers, as well. For example, data can be provided to contain minimum and maximum length string data to verify that neither break their implementation, etc.

Since the factories we use are wired as to which "ISomethingRepository" to return, we have local testing configurations, build testing configurations, production configurations, etc. We purposely are trying to make sure that no team on the project has unreasonable wait times because of another team's implementation time. The largest chunk of wait time is still pr