Let's say I have a wallet backup burned on a CD. In that moment I have 10 coins. And I start to make transactions. Then my hard-drive crashes and in the moment of crash I have 20 coins. When I restore my wallet.dat from CD what I see in Bitcoin wallet? I need to make a backup after each transaction?
Assuming you have not created any new bitcoin addresses from the time that you had only 10 coins, your balance will remain intact (20 BTC) when you load your private keys into another client.
In other words, your client doesn't store your coins locally, it only stores a reference (through your keys) to the unspent transactions that have been received by your addresses which at a high level compose your wallet's balance.