?> Isn't this just a quality of implementation issue?
No, frankly it isn't. No matter how good the implementation of the JavaScript engine on mobile, the mobile device will always be much more limited in processing power than a desktop browser environment. There are those who think that the mobile and desktop paradigms are not (or shouldn't be) fundamentally different in some respects, but those people are incorrect.
Mobile will always have special challenges that desktop may not face. And the faster that mobile devices get, the more complex the scripts that devs will want to shove down the pipes to run on it. You can't just wait around for some mythical future time when mobile processing power is not a limiting factor. The fact is, it's a limiting factor right now, and will be for any foreseeable future. And so we're trying to find ways to juggle the costly operations around to help mitigate the impact.
It's also tempting to just get mired down in this one use-case of mobile JavaScript parsing deferral. While this use-case is a great example of why controlling execution is important, there are plenty of other use-cases for loading a script ahead of time and not using the script (parsing/executing it) until later (or sometimes never).
--Kyle
