In this context, I see interfaces as mostly a psychological exercise. In that sense, it can have some value. Some people think nothing of adding just one more

In this context, I see interfaces as mostly a psychological exercise. In that sense, it can have some value. Some people think nothing of adding just one more method to a class. Sure, it's unrelated, which they (may) realize on some level, but for the sake of expediency, it's going in the class. Some of those same people would pause and think a minute before putting the same method into an interface. They wouldn't feel it belongs there. Sure, in an ideal world, everyone is 100% rational and 100% enlightened and they realize it's really the same thing. But in the real world, people are a bit irrational at times. People often feel that $99.99 is a lot less money than $100.00, for example. The point is, having to include it in an interface can be an occasion for reflection on whether it really belongs there. It's not unlike how when you move, you go through your belongings, and that triggers a process of justifying to yourself whether you really still need that item. In theory, there is no reason why you can't do that at any other time. In practice, it serves as an occasion that encourages the process to actually happen. That said, if you don't through this process of examination, and you just create a Foo and think "I'm supposed to have an IFoo" and then just mechanically copy everything over, then you're not getting any value out of that.