I don't think WhatsApp Web is a ridiculous hack. Instead I think it's quite an ingenious hack to allow you to use a computer to send and receive messages without 1) dealing with the pesky message synchronisation issues that plague iMessage, and 2) having the server store all messages. And your phone only needs to be connected to the Internet, not necessarily on the same network. Regarding the use of QR code, I think it's a clever way to authenticate too; no password to memorise, just a long random auth token.