Make .js extension the default?

[jerrymao15]

Dan Abramov recently made a good point in favor of using .js over .jsx as an extension while developing in React (facebook/create-react-app#87 (comment)). Would making the default to .js be the better option? If the default is .js, the directory option would have to be required until we find a better way of determining what/where your source code is.