Surely at some point I will need to transfer coins from my savings to the spending address ? I will have to count how many times I'm doing it to check if I need to do a new backup. With persisting address, I need backup only once.

Anyway the remark was more about foolproofing the default behavior of the client, lower the surprise effect and improve data loss prevention.

I understand there is a tradeoff between built-in untraceability and having persisting addresses for the sake of simplicity and managing one's accounts.
Right now I don't care too much about traceability of my transactions, since it is still hard to match an address with a physical person. I know others will care and will use a different address for each tx. Should it be on them to opt-in ? Could I at least opt-out ?

With the keypool of 100 addresses created at initialization, I will have plenty of room to segment my wallet for different usages. I could have a spending address dedicated to food, another for books, etc. all within the same wallet.

To sum it up:
- keypools = great. If I backup on day one, I have a hundred of addresses backed up.
- using a new address to send the spare change of a transaction = should be opt-in or disabl-able. Otherwise my backup dies at tx 101 without me noticing it.