I know this is kind of an old topic but for those wondering why they have holes drilled in the bottom of kegs it is to let pressure out while welding.
If you look at bicycle frames you will see they have tiny holes drilled in the tubes where they meet the bottom bracket. This is so the tubes don't explode when they are welding them together. Custom frame builders will weld the hole shut afterwards since that takes only a small tack weld.
The one kegs that exploded while being used for brewing was a fluke, someone forgot the safety holes, the welder was the lucky one in that case.
Still, it is a good idea to check your kegs...