What do you want to edit images in a browser for? Browse with a browser, edit images in an image editing app, not some cobbled together sluggish browser live-app "thing".
:shrug: Photoshop Express is piling up users, and so's the whole cloud computing thing. Doing that without needing a proprietary plugin like Flash or Silverlight would be a win, and with fast JS it can stop being a "sluggish browser live-app 'thing'"...
There's loads of apps that aren't possible in a current browser just because of JS speed, image editing was just the (fairly extreme) example I've seen first hand. Even just speeding up JS-heavy pages like Gmail is worthwhile when you consider that a 5x speedup starts to make these things usable on a mobile device using an identical browser.