Depends really. As long as you don't thrash your car, slam on the brakes, pull over and turn your engine off you don't need one. If your car is stock standard you don't really need one.
Remember the purpose of the turbo timer is to allow the turbo to cool down by letting it idle for a bit (circulate oil and water through it) if the car has had a bit of a thrashing.
No car really needs a turbo timer. You can simply drive sensibly for the last few minutes of your drive or simply let the car run for 20 odd seconds when you pull up (don't have it running for like 5 minutes like some wankers do).
It is just the practicallity of having one and the fact (the main reason I like the Greddy one) is that they usually come with a lot of handy features such as voltage display, lap timer, 0-100km/h etc.
If you allready have one and you can install it yourself, you might aswell.