If you go back and re-read the original post by David, he was talking specifically about the mini-figure 8s, sometimes used by rock climbers as belay devices and emergency rappel devices. All the discussion so far has focused on standard 8s or rescue 8s (8s with ears). Geary outlined the pos and cons pretty well. I have used 8s extensively for short drops, and even wore out a couple as the gritty rope abraded through the anodizing on the 8. One of the biggest drawbacks of 8s is that they cannot easily be used with rebelays, as you have to detach them from your harness to put them on and off the rope. Mini-8s would not be at all suitable as descenders for caving, except in emergency situations, just like Petzl's Tibloc ascenders are not suitable for regular caving, except in emergencies.
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