The other thing that occurs to me that we're not necessarily dealing with now but will happen eventually as more and more pure Trust Members are brought into the system, is the assumption that Trust Members have a similar guarantee to Trust-conveyed intervals that Weeks Owners have to intervals at their home resorts.
Marriott can't just dump intervals from the DC Exchange Company into the Trust so if the object is to make all intervals available to all DC Members, the only alternatives are to either keep everything completely separate, or, dump everthing into the Exchange Company. But doing that, effectively making every interval available on an equal basis to Trust and Exchange Members, could eventually lead to Trust Members not being able to use their Points. Not now, and maybe not even soon, but eventually when the program has matured and there are more pure Trust Members onboard, it's Marriott's ability to keep separate the various inventory pools which will safeguard those Trust Members' usage rights.