I use this feature with a Meteor app and it's really nice. Just be sure to not load a corrupted or crashing bundle, else your user will need to reinstall the app.
Apple is allowing this because JS is sandboxed, so I guess it's here to stay.
I wouldn't be so sure. If this starts getting real traction then it is a huge leap over the native development process. Which ironically is all Apple's fault for it taking so long for them to approve updates.
Having said that Apple seems slightly more chilled now Cook's in charge.
Also, this is react native, so it is only sandboxed in the way any other app is. The JS->ObjC bridge can do pretty much anything a native app can do so I don't think the JS thing makes much difference.
I think really it's in there so people can update webviews of content without having to resubmit the whole app. I don't think this is the spirit of that rule. Would be pleasantly surprised if not though.
Apple is allowing this because JS is sandboxed, so I guess it's here to stay.