`React.createClass` was created at a time when there were no classes as part of the JS language itself, and every library was busy inventing its own class-like inheritance implementation. Now that that classes are an actual part of the language, they're a common standard of syntax and behavior that tools can understand, so there's no reason to keep using a homegrown implementation. In addition, in React 16 the React team will remove `createClass` from the main React package and move it into a separate package, thus shrinking package size.
There's a variety of approaches to manage method binding. See the "Using React with ES6" section of my links list for discussion and information: https://github.com/markerikson/react-redux-links/blob/master...
(I'd normally go dig up some links to comments from the React team, but I'll leave that as an exercise for the asker this time - trust me, you shouldn't have to look too hard.)