It solves a problem for some while remaining infuriating for others.
I think I understand why they do this though. The idea is hat they don't want to store messages in an 'account' like any other chat app. Once you delete a message on your phone it is gone forever just like SMS. Ofvcourse I think Whatsapp can avail it to the authorities if need be but for all intents and purpose the message only exists in your phone and the recipient/ sender.
This is unlike Viber which is just like Google chat or telegram or whatever other service. The "home" of the message is not your phone but the web.