I would say that the y-combinator is interesting to the such a degree that it becomes useful. Sure you won't and shouldn't use the y-combinator in any real world applications. But first off it demonstrates the true power of the lambda calculus to represent computation, and more importantly to the practical programmer is that understanding it will provide a deep insight into the nature of computation, which is profoundly useful if you ask me.
It's a stretch to say that you shouldn't use them in a real application. Combinators in general can be useful for implementing functional programming languages, and those languages can be useful for applications.
That may sound like bickering over a fine point, but I'm just saying that combinators can be more than a theoretical tool.
