I use most of my CS skills almost daily.

I may not be writing compilers professionally, but on the surface, understanding parsing has gotten me out of more than a few tight corners. More abstractly, there are lessens to be learned in designing large applications; and appreciating the difference between a finite state machine and a deterministic turing machine, including the types of problems they can solve and how much of a pain in the ass one is to write compared to the other.