Will we be allowed to use some ES2015 features at some point in the future?
At the moment this what the documentation says and there is no explicit error thrown when we use ES6 syntax but it might not work in IE11. If you are building products in AppExchange better to stick to ES5
The Aura Components programming model supports ES5 syntax and ES6 Promises.
For the most reliable experience, use ES5 to develop Aura components because the pipeline from authoring to serialization to execution was built for ES5. Promises from ES6 are also available. Using any other syntax or feature is not supported, and does not work in IE11.