How much trouble would you be in if your smartphone suddenly broke?
I know some people who keep an address book only in their smartphone. It means that if their smartphone stops working they can't contact anyone. We should realize that it can break any time with no warning.
It is not difficult to synchronize devices, so I keep important data (including the address book) synchronized across a PC, a tablet, and a smartphone. I back up this synchronized PC often, to an external hard drive. I also back up all important data that's on other, stand-alone devices.
I still remember the trouble it caused me the last time one of my PCs broke.