Great, so G+ is yet another attempt to get people to stop using common communications standards like email and instead break the internet up into a set of distinct one-provider-oriented services that can't talk to each other?
The problem is that your social graph on Facebook is probably everyone you've ever met. Google+, on the other hand, feels much more like using email. Circles are kind of like mailing lists which we use to keep in touch with real friends and colleagues. Incidentally, these are also the people we email photos, documents and presentations to.
Circles is at the heart of Google+ because it makes everything they own better. More and more apps are going to move into the cloud and I'm betting that Google+ is going let developers take advantage of this.
I would expect that idea to apply to apps and the sharing of documents as well. If your friend is not using google plus, the document could be sent to their email instead, but the though process would be "I need to send them this document" instead of "I need to email this document to them" or even "I need to send them this document using google docs".