I see you boys are developing your light-weight wallet spec. https://github.com/input-output-hk/cardano-sl/pull/2669/files
Would you consider the use case where someone wants to demonstrate an account balance to a third party? A read-only wallet if you like. Or a Monero view key, to use another ecosystem’s nomenclature.
Martin wants to let Bob see how much ADA he has in his wallet - real time, all the time - how are you going to do that?