Changing that now would probably break all existing users.
As Craig said - "tie nodes together to add functions as a message proceeds through the chain" is one of the key points of Node-RED... each node should do what it does, and do it well, and not try to do everything.
But yes things do evolve over time. Back in the days before it was even called Node-RED payload and topic were about the only things it understood. So everything that wasn't a payload was probably a topic.... (Some "what was I thinking" examples of that still remain - for example some of the database nodes query modes - aargh!). Obviously that was dumb and abandoned early on - but yes in general payload is considered the first place to look for "interesting" data from a node and what every node should expect and produce. Topic does make sense as a secondary piece of meta-data in many cases, but there are so many others that hard coding it into every node doesn't make sense when it is easy to add/remove with a change node (for example).
One thing we do want to look at fairly soon though (post 1.0) - is an upgrade to the inject node so that it can inject more than just payload (and topic), current thinking along the lines of the change node (as in you can add multiple "rules" or in this case properties, with payload and topic as the defaults naturally, so indeed this would produce a complete msg.