Какую роль делегаты играют во внедрении зависимости?

I know that MEF for example allows injecting delegates. However you can also make an ILog interface that has a Log method with the same signature as your delegate. I think it'll be much clearer to understand that the intend was to inject an implementation of an object capable of logging rather than a single log function.

I occasionally use delegates as Anonymous Interfaces - also for DI.

One issue with this approach, however, is that it becomes a little bit more difficult to unit test that the correct Dependency was injected and used in a class, because a delegate instance isn't a type, and sometimes you'd simply just want to verify that a class uses the correct type of Strategy/Dependency.