If you want access to all of your bitcoins in the future, you will need ALL of your private keys (including the keys for the hidden addresses).
Every time you send a new transaction, the wallet will create another hidden address with another hidden private key. Therefore, if you write down ALL your private keys, and then send a transaction after that, you will no longer have ALL the private keys.
You mean you need the private keys to all addresses that contain bitcoins. Otherwise they are lost.
But shouldn't a backup of the private keys create private keys for the change addresses too? If not then this sounds like a security risk.
no he mean that when you send a transaction the client create another address for the rest, it is something that it was done for the pseudo anonimity of bitcoin
so you need to dump ALL the private keys before you want to delete your wallet, and not just those that are associate with your addresses