What is the point of UIWebView, if it's meant to help developers write Javascript "native" apps, and yet it's 3x slower than a web app would be inside Mobile Safari? Doesn't that kind of defeat the purpose of UIWebView existing?

I'm pretty sure Apple is leaving this as it is on purpose so Javascript apps don't start to compete on performance with ObjectiveC apps, and the security excuse they give is just the lazy one to get them off the hook, just like Microsoft is saying "WebGL poses security issues, so we won't even try to add it".

Well guess what. A lot of stuff poses security issues. But you have to balance that out with the features and usability you want to give to the user, and if there are some good and "logical" features you could add like setting other browsers as default, or letting UIWebView apps be as fast as web apps in Safari, then you just have to add them and make sure the security is as tight as possible. Add a sandbox if necessary and so on. Don't take the easy way out.