If we allow collaboration directly on documents without Groups. I am concern on how well the Gossiping will work dealing with the Offline Problem. Groups with their list of members and Sites add an important infrastructure to improve the gossiping performance, thus the Offline Problem.
So if we are saying that a document that has only been published to an Account will be used for collaboration, then distributing the information will be based entirely on the Users Contact List 0r Web of Trust Gossiping. While the user doesnt have enough Contacts, Offline will be a problem.