turbo timer is not really necessary, but it depends a bit on the driver (IMO).

If the driver is likely to drive it reasonably hard and shut it down hot... well maybe its a good idea.

If the driver can be aware that it should be driven gently for 3-5 minutes before shutdown, then its not really useful.

(In my experience (I use EGT gauges on my turbo diesels) if you have been driving it hard, it takes 3-4 minutes of idling for a very hot turbo to cool down to a safe shutdown temp.
If you have been driving sedately, the egt's are OK without any cool down time.)