One reason for this change is to make it possible to use object literals or record syntax where that is more appropriate than function calls. We don't currently recommend it because there's no validation in that case, so you probably want static analysis to catch errors. I would encourage you to play with the idea of using object literals instead of function calls though.
One compelling reason for this change is that in 0.13 you will be able to build components using plain CoffeeScript classes instead of relying on React.createClass. So, in the end, you will be getting some of that bloat/overhead back.
We're definitely not making React depend on JSX. We will continue to support non-JSX and fully support compile-to-JS languages. Unfortunately, that sometimes means a trade-off. In this case trading React.createClass for React.createFactory. Some would've preferred it be the opposite tradeoff but React.createFactory gives us more benefits than the opposite.