I've never really understood the concept behind letting a turbo car idle down before cutting the ignition. Well, okay, I understand the concept, but it doesn't seem very logical.

My understanding is that allowing the car to idle for 15-30 seconds after a long drive prevents the (hot) turbo from being starved of oil when the car is shut off. But, thing is, it will still be abruptly "deprived" of oil when the engine is shut off, even if it's been idling for 5 minutes, and that oil will still be relatively hot, so coking is still (theoretically) a possibility. I can see how you would want to let the engine idle down if, for example, you had just been on the track, but does it really make a difference with normal street driving?