Kind of off-topic, but one avenue where serial numbers stamped on all major bits have been useful is to collectors. I've seen a few that Forgotten Weapons covered that went for a pretty penny at auction because everything had matching serial numbers...

But that's a really good point about liability since there's so many after-market replacement barrels for Glocks. Heck, I've been tempted at times to get one just for the standard rifling (and I don't shoot lead).