There was a post on here about it recent enough, but I don't think I saved the link. It basically suggested that it shouldn't matter what social network your friends are using - you should be able to seamlessly connect with someone who is using facebook from google plus.
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".
But how does Google+ help accomplish that goal? It's a closed platform just like every other has been; for that you need federation APIs (like those of Appleseed & StatusNet) to enable cross-server communication.
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".