If a user has multiple accounts (for example an extra account for insecure connections, or a bot account), the user talk page for the extra account is often redirected so that the posts end up at the talk page of the main account. Also, if a user account is renamed, the user and user talk pages for the previous user name tend to be redirected to the new user name. How will these features be implemented using Flow? I hope it won't be like Wikia's horrible message wall where I can't figure out how to add that kind of redirects.
Topic on Talk:Flow/Interactive prototype
I think that user renames should end up working transparently, for the most part.
What you're talking about is the concept of "Merging Boards". I have a chat with Erik about its implementation; I can't imagine it being that difficult.