Apple is absolutely not engaged in some kind of undisclosed information harvesting--I'm not suggesting it implicitly or explicitly--but it is storing information that a rational person wouldn't expect it would record. As a result, as with Yosemite's Spotlight suggestions, more explicit disclosure to users is warranted.
No regular user knows that, by using iCloud in Apple's approved manner, his or her unsaved documents are stored in iCloud. Many technically advanced users aren't aware of this, either. It's a feature, to be sure, but also carries the same risks of any cloud-stored document, no matter how much one trusts a company's intent and security measures.
Despite Apple recently aggravating the FBI and NSA by making its mobile devices secure against the company's own ability to crack them on behalf of the government, its behavior appears suspect to many because it just works and because Apple controls app-level integration. Dropbox is an explicit folder. Crashplan, Backblaze, and the like copy your data because you install and authorize them to do so.
By trying to make it magical, invisible, and simple, Apple takes away some of a user's agency on privacy unless that user is extremely vigilant. No, I don't want modal dialogs to pop up all over my screen that read, "We are about to store your unsaved changes in iCloud. OK." Rather, I want them to make it clear to everyone where the data goes--and an option to opt out and later change our mind.
In the iCloud preference pane, enabling iCloud Drive could bring a simple message: "iCloud Drive stores unsaved changes for all apps that use it," and two choices: "Keep Unsaved Changes in iCloud" and "Keep Unsaved Changes on Your Computer." A checkbox could allow a later change of heart.
Apple can wrap elegance inside of explicit disclosure and choice. As the company gets battered again and again with avoidable "gotchas," perhaps it will give its users more credit for their ability to make a choice without removing the sparkle from OS X.
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