In reply to David Alcock: As the John Cook in the OP here are my stats, 6ft tall, AI either 1.0625 or 4.5". A positive ape index can be useful if you know how to use it. Flexibility helps this use. It is good on reachy routes, but in chimneys and under roofs can be a disadvantage. Because my arms and legs are long, and I am flexible enough to use my AI, I can outreach some crux moves, and bridge where others cannot.
I have a friend who is taller than me, with a bigger AI, but because he has very muscular shoulders and is inflexible he cannot reach as far upwards as I can.
Pull-ups and lock-offs are harder as a result of the long levers involved.
I have been known to climb 'short', as two of the ladies I used to climb with were short and always complained about my reach. Indoors I was not allowed to reach further than they could, which worked wonders for my footwork.
All this still does not mean that AI has any relevance to how good a climber you are!