A possible resulting disadvantage is an ever growing mailbox. In a forum, one can join the discussion when convenient. Some forum software allows members to mark threads of interest to have notices of new messages mailed to them. Chats WebBoard also provides synchronous textual chats. That is, participants can sign on at a particular time and have running text conversations with others.
Chats are perhaps of less use in courses.
Chat sessions scroll forward in real time, so session information disappears as the chat progresses. Chat information can be archived, but the dialogue nature of this method means much of the content is not intended for posterity. Its forte is in the moment.