Well, here's a few quotes from various website:
http://ca.dsm.org/FAQ/2gtt-faq.html
The basic accepted concept of cooling the turbo in your engine, is to allow your turbo to cool off properly when you stop the car, by feeding it a steady stream of fresh oil, and thus likely preventing coking of the bearings (gunk buildup). Most manufacturers of turbo-charged vehicles recommend idling your car after you get where you're going, for at least 30 seconds, longer if you have really been boosting it...
http://uktoyotaestimasite.tripod.com/My%20Estima%20Review.htm
...Toyota recommend that an Estima be idled for a period of time after moderate to hard driving to allow the oil/water to take heat out of the turbocharger...
http://en.wikipedia.org/wiki/Turbo_timer
A
turbo timer is a device designed to keep an automotive engine running for a pre-specified period of time in order to automatically execute the cool-down period required to prevent premature turbo wear and failure...
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Now onto my own justification. For an engine, when you are running it, the engine oil is not really the one that cools the engine, unless you've got an oil cooler installed. The main cooler is the radiator system. That takes heat away from the engine, and the fan itself kicks in for additional help when outside air is not enough. The purpose of such system is to keep the engine's heat constant at the most optimized level all the time while your engine is on. Doesn't matter if you drive 400 km non-stop, or 5 km, the coolant system's job is to keep the engine's heat stable all the way. Therefore, after a long drive, it won't hurt the engine if you shut it off immediately. WHy? Simple logic.
You said the job of the TT is to cool down the engine. Now let's see, why? Cause when heated up to fusion point, the pistons will melt and blow the engine up. Right? So what creates the heat? The combustion in the engine, correct? Now if there's no combustion in the engine, where does the heat comes from? NOWHERE! So why idling the engine for 30 mins is gonna cool down the engine better than just shutting it off and let the atmosphere do it for you? No combustion = no heat.
The engine doesn't need a long cool down period after a long drive. After a HARD drive yeah, its good to leave the engine running for a while.
Turbines get heated up very fast when you're driving it pretty hard, and unlike the engine, the consequences of not cooling it down properly is pretty severe. The compressor assembly might be wrecked if its hot, and no oil is fed into it after the engine has been turned off. That's y most manufacturers recommend a cooling period after a hard run. For normal drives you can do it the same way as an NA - turn it off once you've parked your car.
No offense, just providing my side of argument.