Many file types can be opened by several different applications, and using the Mail app to transfer them from one app to another as we do now is simply an awful, awful kludge. If I'm looking for a file on my iPhone, I'd much rather have a central document repository to look through than to have to dig through a bunch of emails to find the attachment in question, or use a bunch of unique app interfaces with their own document management implementation. This limitation is so obvious that I'm surprised anyone would actually think the status quo is acceptable, let alone advantageous.