The point of the machine was simply counter-example to the notion that unbendable arm is 'not a trick'. I think that it is a trick unless presented with disclaimers, as people are led to think that they are applying their whole weight to the bending, or an otherwise massive amount of force. In reality, they are essentially helping to keep the arm extended with their shoulder and limiting themselves when pulling on the elbow.
I had no problem with the exercise when it was presented to me, because my teacher said outright something like - this isn't as amazing a feat as it may seem, it's just an exercise to show you to extend your arm properly.
I don't really feel qualified to say how useful it ultimately is as an Aikido drill. I just thought the jargon being used to describe it sounded excessive.
I have never liked the "unbendable arm" trick (I've mentioned this before) because there are too many ways to come close to doing it, so everyone I know claims to be able to do it, even though most of them are doing it quite differently from each other. Even at the Ki-Society workshop I attended in December, I could feel that different yudansha were doing it slightly differently, in many cases.
A better example of, at core, the same thing would be to take a steady push to the chest and ground it. In reality, the "unbendable arm" is just a variant of that exercise, when done correctly. In fact, ALL of the ki demonstrations are, at core, variations of either that exercise or the "unliftable" exercise.... or a combination of the two. But watch out, I said that glibly and glossed over the fact that there are some sophisticated extensions of the 2 core demo's I just mentioned.