Why couldn't you maintain a local cache with JavaScript? Chrome and Opera already support the Filesystem API and it's only a matter of time before it ceases to be a working draft. But even discounting that API (which allows you to set a user authorized storage limit your application has read/write access to) there are already ways to cache > 5 MB of data in the browser; they're just hacks at this point.
If the FS API were standardized and broadly adopted then yeah, it wouldn't be an issue. I've never gone around trying to store greater than localstorage on all the browsers though to know if it would work.
