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In short, bitcoin core isn't a deterministic wallet. It generates a random private key/address pair each time you send change to yourself (in reality, iirc, it calculates about 100 pk/addresses in advance).
wallet.dat holds these private keys for you. You can export one of them, but if you don't export all of them, you might lose coins.
Best thing is to do a regular backup of your wallet.dat, that way you're sure all your private keys are backupped.
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Pre 0.13.0 this is true. As of 0.13.0, it has HD functionality, just to be clear. He didn't say what versions be was using, but if he created the wallet.dat with 0.13.0, then it could be HD - seems unlikely though.
I'm using the latest version of bitcoin core, also 12W7RgjugzpULqwfq1tCzdU89LZnvVvJVw was the only key in my address book so just backing that up doesn't save my coins?