The obvious application for saving files though would be things like keys for UIs like RiotWeb. Saving keys in the cache isn't a good idea.
If it was restricted to that type of use I'd be ok with adding the ability to access the filesystem. Anything much beyond that just becomes an attack vector.
@bob I've heard some people say that loosening the restrictions on accessing the local system was what led to Java applets' disastrous last years (and their eventual expulsion from browsers).