If Safari team messes up their rendering engine, then iOS update, and it is broken for every iOS browser...
My point is that people are really stretching the term "skin". Bookmark syncing, tab & window management, history management, security management etc. all are features of a browser, and they have a major impact on the UX of that browser. They are WHY there are still users of Firefox or Chrome on iOS! They're not "skin" features, which typically imply minor look & feel related items.
My point is that users don't really care about the common rendering engine. It doesn't have much impact on the day to day UX.