No, they are not essential..but they aren't a pointless toy.
I feel it's different for people like us who know about things like this and how it's important to let the car idle for a bit before switching it off, but Joe Bloggs doesn't and fitting one of these may be a way of prolonging the life of their car.
Yes if they don't know or adhere to this it's their fault, but perhaps they just need a bit of re-educating about turbo cars. They may never have had a turbo before and treat it like a norm asp car.

I work with two people, they own a new golf gti turbo and one's got an ST Focus (which I found out last night). I know the golf owner lets the car warm up before boosting it but unsure of post-drive idling. The ST driver's habits are unknown but I do know he doesn't drive it the 2 mins to work for the right reasons, as he told me last night.

I don't think they know about turbo-timers and could be damaging their turbos through not knowing. If they had a t-t fitted then they wouldn't have to think/ remember to leave it for a short time before switching off.

As I said at the top, no they are not essential but may benefit those who aren't as clued up as we are.