... slightly off topic, but do you have a good resource for learning Arrows? It's on my list of things to do, but I haven't dug around for a good example yet.
In terms of using "them", the HXT documentation made it pretty clear for me. (I put "them" in quotes because things like Arrows and Monads are really adjectives, not nouns. When used as nouns, people generally understand what you mean, but this confused me for a long time and I try not to confuse other people. Consider it my version of a "Monad tutorial". :)
In terms of theory, I found the "Applicative Programming with Effects" paper and the "Typeclassopedia" very helpful.
I started doing a real implementation of these data structures in Perl this week. (I say "real" because people have "added Monads to Perl" before, but they didn't really add Monads, they just added a "programmable semicolon".) Anyway, in doing so, I saw in great detail the relationships between the various types (specifically "liftM2 ($)" and "<*>"), and tried to generalize things as much as possible. The result was something like Arrows, and that helped me understand the "why" in addition to the "how". Writing test cases was also helpful to me; using these esoteric stuctures in "real code" helped me build up the intuition needed to advance farther.
Anyway, it took me three readings of the Applicative Programming with Effects paper to get Applicative Functors, and after that, Arrows were pretty easy. So I recommend that paper, and lots of poking around in ghci.
To bring this back on topic, yeah, it would be hard to teach this stuff to Joe Average. But a language that allows it is still important to develop. (I also learned firsthand that language design decisions to make it difficult to implement certain features. Implementing the equivalent of "instance Monoid b => Monoid (a -> b)" is almost impossible to do cleanly, due to the polymorphism of "mempty"; it's defined in Haskell as "mempty _ = mempty", and in Perl, we discard the type information before we have a chance to pick the "second" mempty.)
