This move by Microsoft was expected and logical. Microsoft has transformed the last couple of years into more open-minded company. They have dedicated teams of active open-source contributors now. Their whole .NET ecosystem was open-sourced, they released several open-source tools like Visual Studio Code and TypeScript, now Charka. Their virtual academy - MVA has grown exponentially.
Other than that, competition is always good for the industry, especially in our field. Competition drives more innovation, which is more than welcome for us - the middle users.
So, is this pull request a good thing - totally. Node.js popularity rates are increasing more than ever. Node.js is being used as a platform from IoT devices to large scale projects. Microsoft wants to influence that markets as much as they can. This is their commit all about. Other than that, they've included Chakra Shrim - a layer on top of Chakra, that implements the most essential V8 APIs, which ensures a transparent switch for other addon modules written for V8.
Performance wise, Chakra is showing an outstanding benchmark results (keep in mind that this tests were ran by Microsoft on Windows machines). V8 still outperforms Chakra in some categories though.
The current state of Node.js ecosystem gives a lot of room for improvements and companies are starting to perceive this opportunities. It's Google move now.