We designed React Native to run with any JS engine: jsc, v8, (wk)webview, inside of Chrome/Safari/Firefox, inside of Node, ... We released it with JSC today mostly for convenience: we had JSC setup from iOS and it works. But we're open to ship with a different engine if there are performance benefits.
As I recall, they announced that alongside React Native in January. I believe it was to avoid subtle differences between the JS executed on various platforms (e.g. to prevent a component from using a feature supported in V8 but not JSC).
There are a bunch of other benefits of this, e.g. we use same API on both platforms to hook up profiling. We definitely want to experiment with other JS VMs in the future though.
