Just a "mind expanding" language chosen to be as different from lisp as possible (I am comfortable with static type systems and OCaml/Haskell for example). Also, I'd like (at some later date) to grok Factor.
I can't answer for plinkplonk but my guess is, for CS learning purposes, Forth paired with Lisp is a combination like pizza and beer (as opposed to pizza and sandwich).
very interesting analogy but I still don't get the picture :-)