Functor pattern is certainly something else than functor in category theory.
By "useful" I mean "useful in getting new results or new, better proofs". By this metric, singular homology is certainly useful, because it enables us to create clean and elegant proofs of some highly non-trivial and interesting facts, but I cannot imagine anything more useless in real life.
I had no idea the gang of four had appropriated that word too. My bad. I really did mean the category theory functor, not whatever its other meaning is.
However useful or not category theory is for producing new proofs, I found it enlightening and am glad this was posted.
By "useful" I mean "useful in getting new results or new, better proofs". By this metric, singular homology is certainly useful, because it enables us to create clean and elegant proofs of some highly non-trivial and interesting facts, but I cannot imagine anything more useless in real life.