Interfaces are a critical part of Dependency Injection and will allow you to Mock the functionality defined by the interfaces during Unit testing. I completely disagree with the idea that they are redundant if you think one class will ever implement them.
I have numerous applications where I have defined an interface and only have one class implementation of said interface. The power of interfaces lies in the ability to define abstractions in our programs and create dependancies on said abstractions to promote loose coupling. Interfaces aren't just used to achieve polymorphism. They allow for flexibility in your code...