Just a small question. Wouldn't this make it easier for an unauthorized individual to make a copy of the wallet and then use it at an arbitrary date in the future to access and steal any coins in it?
Yep. That is the compromise. The current wallets unsteal themselves.
My arguments:
(0) In spite of recent concerns, I think that loss is a greater risk for most users than theft.
(1) People frequently reuse addresses today, this eliminates the unstealing advantage.
[And, in fact, someone on IRC was just asking about changing the client to send change back to the original address in order to make backups more effective]
(2) Encryption makes theft less likely than it has been
(3) Thieves know about the unstealing property and will already act fast enough if coin is available. If you're about to receive a lot of new coin you could start a new deterministic wallet. Also after compromising your machine once the thieves will likely leave backdoors in any-case.
If the interface allowed you to have multiple deterministic wallets at once (or just multiple key-heads in a single wallet) you could periodically trigger the creation of a new one in order to get the forward privacy, and you could do this in time with your backup schedule so you're not left surprised by a backup that failed to grab all your addresses.