All JS engines make different tradeoffs, so they are better at some things compared to other engines.

For example, Chakra has had full asm.js optimization support for a long time now, while v8 still doesn't (but it's coming).

We now have 4 high-quality JS engines that are cross-platform and open source (v8, SpiderMonkey, JavaScriptCore, and now Chakra). This is great!

The Node.JS project has a problem in that it has never had a specification - only a defacto implementation based on an ever-changing V8 engine with whatever ECMAScript compliance that V8 had at the time. Recently Google made big changes to the V8 API that broke Node Buffer in a rather large way and the Node devs had to do handsprings to put together a less than optimal workaround to get it working with V8 ArrayBuffer. Node.JS is at the mercy of Google's V8 engine which is designed first and foremost for Chrome.

ChakraCore did the impossible in that they've mimicked the V8 C++ API to make it appear to Node as V8. But this trick will only work for so long as Google continues to change its engine.

It would be better for Node long term to have a JS engine independent C ABI. But the core developers of the Node project are resisting this idea as they believe Node is V8 only. I don't think that Node will ever officially merge Chakra. They just want it to go away.

When MS announced a node.js version backed by Chakra instead of V8, some of the comments (including one from me) addressed some of the reasons that having multiple JS engines is a good thing.

https://news.ycombinator.com/item?id=10932182

Competetion.

It's an engine -- it shouldn't bring much that is different.

Having hundreds of frameworks, platforms, etc isn't great, but having at least one alternate is awesome. Minimally it allows for a larger collection of minds to deal with hard performance problems, to each take different approaches, and to potentially standardize on a solution after sufficient time to prove out solutions.

Off the top of my head: better performance on some workloads, better standards support in some cases, not being controlled by Google.

If you happen to care about those workloads or those cases, or have a company policy to not use or minimize use of Google software (which some places do, especially ones that think they might end up competing with Google), these are pretty useful features.

Sometimes faster, better ES6 support (to hopefully spur V8 to complete theirs), etc. Nothing big, but a little competition is nice.