Collaborators must be able to reach the message store over some kind of network. In addition, a gateway to regular e-mail products can let collaborators who don't have full access contribute.
People make mistakes.
It ought to be easy to relocate a misplaced message or delete completely irrelevant ones. Actual messages don't always fit precisely in a threaded hierarchy. For instance, a single message may include two or more topics. It should be easy to duplicate a message and insert the copies at the places where they fit. For large, ongoing threaded conversations, such as those on commercial online services, the day will arise when the space that the system has available runs out.