The whole point was that in general, something made of aluminum will last longer than something made of plastic. Sorry that point wasn't clear enough.
Sorry, since we are STRICTLY talking about smartphones here, you're being incredibly unrealistic. We all know without a shadow of a doubt, the absolute majority of us replace our phones every 2-3 years. Guess what, the GS3 and every other smartphone made of polycarbonate will last and endure daily use with drops here and there for much longer than we intend on keeping them. I don't know any polycarbonate phone with a shattered back as you put it. Polycarbonate doesn't shatter. But I do know a whole heck of a lot of iPhone 4/4S' that have shattered glass backs, since glass shatters. And that didn't seem to stop you from buying one did it? Did you shout from the hill tops of how bad a glass backing is easily shattered on a mobile device? I'm guessing you couldn't care less. But as stated earlier, most of us use cases, so the whole argument is moot for a majority of us.