Values in the middle indicate that I'll have a conversation and probably not budge, with a chance of being totally convinced.

Ah, so it's more like the probability that the intelligence in the box is over the threshold required to convince you of something. That makes sense.

But I am now convinced that the whole idea of boxing is stupid.

Agreed. Everything you said, plus: if you think there's a chance that your boxed AI might be malicious/unFriendly, talking to it has to be one of the stupidest things you could possibly do...