For the following reasons I never lock mine:

1. A locked suitcase indicates that there may be something worth stealing inside the bag.

2. A lock, no matter how strong, is pointless on a zippered bag. Most zippered bags can be opened with a ballpoint pen and closed with no damage to the lock, bag, or zipper.

3. At times, a bag may have to be opened, without the owner present, for a security inspection or a customs examination. In those cases there is a good chance the lock will be cut off.

I simply avoid putting anything of value in my checked luggage.