[0xffff2]

I'm confused by your comment... I'm a recent convert to iOS from Windows Phone, and I've just kept on using OneDrive for my documents. I'm not even sure what "app-to-app sharing" means. Dropbox definitely isn't indispensable to me.

[kuschku]

For example, creating a file in one app, editing it in another app, sending it to a third app to upload somewhere else.

That's hard to do without a filesystem or proper Intents.

iOS has neither, so people use Dropbox as filesystem.