The thing the differentiates WebKit from IE in a good way is that it's completely open source and it's used and built by several competing companies working together toward a common goal.
It's also behind both IE and Firefox in various ways. You're right that it would be a better dominant engine than IE was, but it's still not as good as having a nice three way competition for browser share.
Even still, jQtouch (at least, I haven't messed with the Sencha thing) is very iDevice slanted. There's "webkit" and then there's "webkit on iOS", and most tools are targeted specifically at "webkit on iOS".