I don't see the point of targeting webkit only for a mobile app, when mobile opera still has such a huge market share. I can see a client thinking only webkit matters because they are their minds are wrapped around buzzwords and iphones. But that is not realistic thinking - that is the client having their heads in the clouds. They won't know better than a developer, so we can either entertain them and make something target bleeding-edge only, or actually make a mobile app that more devices can see. In this office it seems like everyone in the world has a phone with a webkit browser right on it, but I know that my personal community is not the world.
I can actually code JS fine without a framework. However do I want to keep up to date with all the idiosyncrasies of a growing number of browsers? Do I want to rewrite every function that exists in a framework, such as getting the height of an element or the viewport? No.
The speedups in frameworks are realizations of better ways to do things that a group of programmers came up with - which is almost definitely going to be a better way of doing things outside of a community. That argument would only work if your functions were always as fast as possible at start. Don't forget Chrome was already the fastest browser out there for JS, and they still have improvements of 50%+ in updates.