Why are 3 keys a must? Lose your keys often? I've never lost a car key
I've never lost a car key myself either - but I have on occasion forgotten them somewhere (say, at work when I bike) while the second set is out.
Incredibly useful for road trips too.
Note that more than 2 people regularly drive the cars.
And, if a lost key were to happen it'd cost me a minimum of $200 to replace so this is an insurance with the added convenience :p