It's not that making native-looking apps in plain JS isn't worth it – it's not possible unless you're only targeting iPhone.
There may be a place for jQuery, but stay away from Sencha Touch. It's a huge library, and barely runs on any device but an iphone. Don't be fooled by their slick marketing and examples.
Essentially, trying to replicate a native app in JS is a losing battle, unless your app is extremely simplistic. The majority of smartphones are simply too underpowered to replicate a native experience.
Aim for a site like http://m.twitter.com instead of trying to replicate a native experience. Test on real devices you intend to support as soon as possible, even if you've barely written the app.
I grok what you're selling, but looking at JQuery, and especially Sencha Touch, I'm asking myself: how the heck could I make a native-looking app in plain JS without having to dive deep and beating the hell out of myself with all this crap that's already been solved?
Are you saying that the extra UI snazzle just isn't worth it? Or to buck up and start trying to replicate huge pieces of frameworks on my own?