Is a key used for every transaction?
So if you have gone past 100 transactions and you have an old backup - it's usless because you have gone beyond those keys?
Am I getting this now or no?
The default client generates a new key for every transaction, yes. However, the old keys are still the ones opening the vaults to the transactions they were part of - so it would only be "useless" for transactions using newer keys that weren't there when you made that backup. Of course, newer backups would take care of those as well.
Personally I use a tool that makes new encrypted backups whenever the wallet file changes (=new transactions/keys), and I can travel back in time to any previous wallet version.
The official client could indeed explain how this works in a better way.