I'm concerned with how your wallet, the piece of software that you are explicitly giving access to your keys, stores keys.
Its this piece that I think should be standard, secure, and portable.
I disagree.
In other words lets say your wallet protocol was widely adopted.
I should still be able to try out different wallet management software. How would you move the keys between providers or ensure that you weren't just putting your keys into some black box that now has you locked in?
You would export your keys from the old Wallet software (using the Wallet protocol), and then import them to the new Wallet software (again, using the Wallet protocol).
A standard key storage format for keys and only keys would be needed.
The format wouldn't be concerned with balances, etc, because the software accessing the key would assume its a lie and verify for itself anyway.
A standard for communicating keys is needed for various purposes, including export/import, anyway. This can work just fine for the example you gave. Requiring a standard
storage format can prove to prevent innovation in key storage.