As an exercise, yes it's a parlour trick. As part of an entering move, very useful, mainly because of the effect on your upper body rather than anything to do with the arm itself. If you're extending "unbendable arm" properly, your shoulders and neck are relaxed and you're not fighting yourself during the technique. Simplest application I can think of to show this is a basic nikkyo with a cross-handed attack (from static).
Erik, I want the training video (do you do VHS?)