I have never understood the origins of this phrase. And I'd imagine that most people who use it don't actually understand it. Yet they continue to use it. Grumble, grumble, grumble.
After reading The Grammarphobia Blog, which gives a clear etymology of this phrase, I have a new appreciation for it...but that doesn't mean I want to hear Joe Schmo use it to motivate someone on a reality show. Nor do I ever want to hear "throw me under the bus" again.