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.

It seems reasonably fast for my purposes so far, and the expansion rule is just a one-liner: https://github.com/chkoreff/Fexl/blob/master/src/Y.c