But then you need to do so for all your addresses, and these get added on every transaction outbound... back to backing up wallet.dat, I guess, and that is exactly what the average user (heck, even the more technically inclined) will fail to do. I see the solution I proposed (or anything else, it's just an example) as something that could be done so it's a backup once and forget about it, although obviously for a limited time span, but this could be then supported in an automated way by a backup provider for a nominal fee, which I'd be glad to pay so I keep my bitcoins safe *and* still control the client, so I'm sure it's not all in some other server like mybitcoin. An alternative would be for mybitcoin to provide the private keys to the wallet so I could import them on my local client, basically using a single mybitcoin address as the 'savings account' and if in the future their server goes down in a big ball of fire I still have access to my bitcoins.
Backing up would thus be transfering coins to that account. Granted, not backing up, but keeping bitcoins safe, with minimal user knowledge needed for the process.