I have four projects in my Visual Studio solution (everyone targeting .NET 3.5) - for my problem only these two are important:
Including all referenced DLL files from your projectreferences in the Website project is not always a good idea, especially when you're using dependency injection: your web project just want to add a reference to the interface DLL file/project, not any concrete implementation DLL file.
Because if you add a reference directly to an implementation DLL file/project, you can't prevent your developer from calling a "new" on concrete classes of the implementation DLL file/project instead of via the interface. It's also you've stated a "hardcode" in your website to use the implementation.