When a user wants to add a co-worker to an email chain, he/she needs to reply all with the new recipient added. For the email recipients, it is a waste of time to open an email and simply see that a new person was added.
Instead, if Outlook/Exchange could provide a method to add a user and save it on the server so that the next reply all would automatically include the new recipient, it would save time. An alert similar to "you are not replying to the latest email in this thread" could be sent to the new composer, say "a new recipient has been added to this thread"