> My day job is coding an MVC app in Rails / Ember. Fine, I won't be able to get past Chapter 11 in a book, but what am I REALLY missing out on?

Being focused on Ruby (or any dynamic language) makes the value harder to see, perhaps, since a lot of what HKT get you in a static language you don't need in a dynamic language -- its stuff you lose going from a dynamic language to a static language with an insufficiently powerful type system in exchange for typesafety. HKT let you have your cake and it eat it too (that is, lets you keep powerful abstractions that operate on classes of related types, while remaining typesafe.)