Vern you're right. The diaper MIGHT not stay intact 100% of the time. But there have been LOTS of examples where it didn't come apart and did indeed keep the oil contained.
Just like a roll bar won't save everyone one from every wreck, but it doesn't mean they don't help often times.
I had a buddy that killed a 440. Windowed the block /killed the pan. Not one drop off oil under the tires. Diaper stayed together.