If you read HMS' article about his trip to Valtos Uig, Lewis in Rib International, you'll read a good reason for using a Coastkey.
It could be a good way of providing angine stop on a cabin boat cos it would allow unhindered movement in and out of the cabin and around the boat.
Especially useful if you're single handed, you'll be able to handle warps, anchor, fuel cans etc. without stopping the motor or removing the kill cord.
The Coastkey info says that it'll trigger between, 3 & 8 metres, that may be a bit limiting.