If You Ride
Or if you run or basically do anything that puts you away from home and in harm's way (e.g. being on a road during any one of 24 hours in a day), then a RoadID is a good idea. I have one on my cycling shoes but it has my previous address on it. So I've just ordered a new one with my current address and Lycurgus's phone number. I didn't see much point in having my phone number on it since, if I am in need of emergency attention, there's no one at my house to receive a call. I also included my blood type and that I have no known med allergies.
What else is there to say? Apart from the old saw about "in emergency contact: A Doctor!!!"