[comex]

You don't actually have to use iCloud for what I described. I mentioned iCloud Drive because it's built in to iOS and OS X, but any app can register as a document provider, which allows it to be accessed from document pickers (what I meant when I said "can open files directly from iCloud Drive" - it's not only for iCloud Drive) and to be an icon on share sheets. For example, Dropbox does so. This is an iOS 8 feature, for the record; it's just the iCloud Drive standalone app that was new in iOS 9.

You do have to use iCloud for some other things, such as full system backup, integration with the builtin photos app, etc.