Thought I'd revive this thread, because it interests me a bit, and because several good points are raised.
The "outside-inside" kick is a pretty deceptive title, and for good reason -- it's a pretty deceptive kick. Done right, the foot comes straight up, as if it would be an ahp chagi (or ahp podol rigi), instead of, as beginners do, going out first and coming in a circle inwards. The difference comes in the hook at the end. Whereas an ahp chagi uses its power on the way up, a pakeso anuro chagi (I'm giving up on spelling here...) comes up fast but then slams down hard. It's tricky to use, but very useful.
I'm going to echo what upnorthkyosa said about using it to block a punch/knife. If you're pretty durn good, go for it, but it's primarily used for coming down as a strike to the body. Taking into consideration the range issue, if you see your opponent coming at you with an attack, say a roundhouse kick with the right leg, a good counter would be to bring up your left foot and reach in to slam that person on the face/shoulder before that kick gets any momentum. With practice, this is feasible (I've seen/felt it done). The beauty of the outside-inside crescent kick is that the crescent part is all on the way down. Smash the opponent on top of the nose, down on the collar bone, what have you, you now end up with your foot still in a face-kicking position.
So, to go back to a defensive use, if your opponent comes at you with a punch, or even, to an extent, a kick, you can use a quick outside-inside kick to move the attack aside and smash your opponent in the face with a round kick/side kick. TSD is a kicking art; no doubt about that.