I would add the question, why does the encryption not encrypt the whole wallet? The way it is now says "It is ok for everyone to see all the transactions and addresses, but he is not allowed to send." Why is that? Every bank account encrypts all. And you dont let someone look into your fiat wallet normally too.
So why is it ok with bitcoin wallets.
A side effect would be that you don't have to enter the password each time you want to send a transaction. Though a timer should be implemented in case you let the wallet open and leave the computer.
This would be a massive pratical loss of security for most users.
With the current configuration, entry of the password is authorization to send funds. You can't send funds accidentally. No misclicks or fooling around, or someone telling you to press something over the internet can result in you sending funds without realizing it. If someone grabs your computer out of your hands or uses it while you've stepped away they will not have access to your funds.
The keys (or their equivilents) are not kept in memory; so if your wallet is encrypted and your system compromised while you're aware of it, you can rescue things by copying off the wallet and NOT entering the key after the compromise. (I've spoken to two people who mentioned being rescued by this, in fact.)
You are not forced to enter the keys at every startup (further exposing them to key loggers or people seeing you typing them). And you are not discouraged from entering in a strong password because you must type it frequently.
Meanwhile, there are two costs: One has basically no marginal cost: someone who compromises your computer (or steals it-- if you're not using disk encryption--) can tell what addresses are yours; but they can already tell this from your debug.log, swap file, browser cache, and 1001 other small places that data about your activities on your computer are left around.
The second is that backups aren't private and probably should be encrypted or stored on an encrypted disk. This could be addressed by seperately completely encrypting backups. ::shrugs::
The comparison to a back isn't really a good parallel. Any computer in the world can connect to your bank website. Access to your wallet metadata is gated by physical possession of the wallet file, which one normally can't get without access to your computer or your backups and if someone has access to that you've already substantially lost your privacy.