Of course not - I haven't practically applied Turing Machines either. You'd have to be mad to implement a functional programming language purely in terms of S and K - which of course is what I did for my final year project and got a 1st and a prize!

What my encounter with the fundamentals of Computer Science did give me (apart from the necessity of implementing things like garbage collectors) - was an abiding interest and love of the mathematical foundations of computation and with maths as something interesting rather than something to be endured.

That interest did lead to me doing postgrad research in a control engineering group, which did lead to me co-founding a startup.

So did I ever apply S & K in a practical circumstance - no. Am I glad I took a turn down the more mathematical side of CS - absolutely.

The use of the Y Combinator to enable true memoization of a recursive function has been discussed on the 'net a number of times.

I used the mockingbird version for a very specific purpose in production a while back, although that was more like corecursion than recursion.