The assumption behind using cable locks is that you are always going to be in close proximity to the bike while on tour and you simply need some way to keep a thief from carting off your bike while your back is turned. Cable locks are lightweight and convenient, you can thread them through pannier handles, and at the very least they force the thief to cut something before they can make off with a wheel or a whole bag. With a u-lock or a chain, your bike frame might be lock safe, but your panniers and your front wheel will still be vulnerable unless you bring cables to secure those items.