because the message can be sent to different devices (resources) the user is connected with. This is not accidental complexity like you make it out to be. It is needed and wanted.
Or are you advocating a binary protocol? Personally I really like the messages exchanged with xmpp: they are easy to understand and easy to craft. I think the success of http too, lies a little bit in the fact that it is human readable.
Yes I am advocating a binary protocol. If you want to have decent battery life, then you need to be efficient. This is the tradeoff you need to make. for something like text, I really don't understand peoples fear of binary protocols.
As an aside clients have memory nowadays, there is no reason to send the full ID for the sender in each message (unless of course you are doing encyption, but even then, once you've exchanged keys its moot) a client needs to keep track of buddies that are online, so this data can be cached, and a local ID used instead.