Loading more than 5mb on a phone could take a noticeable while, so it's not something you want to do every page load.
Data reads just need to be asynchronous and read at time of request. The standard can't worry about how people might mess up using the feature (not security but performance). A program on my desktop takes time to load and read data. A web app shouldn't be any different given it has the same functionality as the desktop app. The web world needs to stop worrying about performance so much and just work on functionality first. Don't limit functionality because a fear of performance. Of course work on performance but if I can't do it at all, then it's worthless.
The web world is used to "pages need to load fast". They only need to load fast when they are incredibly simple and "dumb" pages. When the world of actual useful/powerful web apps start coming, people will expect and be OK with load times of a web app.
When the web apps start being more like http://www.youtube.com/watch?v=HCwH9BGgtdQ
it's OK to take a little time to load. Users will put up with it because it's so much more powerful than some semi-dynamic web page.
I think this is a mind shift. Powerful web apps need more access to hardware and the standard should be helping with this.
I'm excited to see your work in the storage space to help fill the gap though!