Take a look at WebKit.org for a second. They're moving fast, they're changing things up at an insane speed at times. On a Mac, just throw Nightly on there and see just how much better it is to be able to run the latest build. Weeks and weeks pass by with no update to be seen on iOS, unlike OS X where you can just run the Nighly like I said.
I'd like it Safari was updated on a monthly basis, and likely not tied to a version update, but that's not likely to happen.
I've been using Webkit Nightly for several years on OS X, it often goes through long period of instabilities. There was a period of nightly builds that crashed upon opening for me until it was fixed 2 full months later. The webkit devs focuses on adding brand new experimental features right away and then spend several weeks to clean up after it. So, no, don't compare Webkit Nightly against stable Safari that rarely has issues.
Apple doesn't rush this because it risks the chance of crashing every single app that relies on the same Safari rendering engine. What Apple could do is:
1. Split between Safari and webkit framework, the stable webkit framework remains the same like Safari on iOS right now for all iOS apps to use. Release a separate Safari app
2. The separate Safari app should be in the App Store, ready to be updated any time Apple wants but there needs to be a way to revert easily in case things go wrong.
Considering the amount of QA issues they've been having lately, I rather they focus on stability and not rush through things just because Webkit happens to be slightly faster.