I've tried to logically reason this one out, and am probably missing something blindingly obvious, but I can't for the life of me figure out the benefit to either Disney or the owners to "use years" and the "rule of four". There's obviously a lot of overhead to having to track and enforce it, though, so there must be some benefit to somebody.
Can someone clue me in?
The rule of four is pretty easy - Disney needs some amount of time to rent out the room.
For simplicity, let's imagine there are no "use years" and everyone's points are good during that calendar year. When someone trades out of
DVC, Disney (specifically the Buena Vista Trading Company (BVTC) ) gets their points, and uses those points to rent DVC rooms for cash. The cash covers the cost of paying for whatever the person traded for, be it a cruise,
Disneyland Hotel stay, etc. But those points need to be used before the end of the year, because the next year's room-nights are already spoken for. Similarly, when people bank points, that leaves potentially unused room nights, which Disney rents out. Again those points need to be used during that year. So if people suddenly banked or traded out two weeks before the end of the year, Disney could be on the hook to move a lot of room nights for cash with very little time to do so.
The bottom line is if you do anything with your points that doesn't involve staying in a DVC resort during those points' normal year, Disney may end up having to rent out the room-nights that weren't used (though not always, since borrowed points might balance out banked points). Since they have to do so before the next year is up, they want enough time to get that done.
If in fact there were no use years, that would mean there would be a flurry of activity where everyone was trying to snag last minute bookings in December. In addition, some people's standard vacation period might not work well with the calendar year, because they'd be constantly traveling inside that 4-month period near the end of the year, and would be in trouble if they had to cancel. So Disney created the "use year" concept to spread the demand across the whole year and allow people to position their 4-month "no-bank no-trade" zone across the part of the year that they're least likely to travel.
It's not essential that there be an even number of use years, or that there be one for every month. Disney decided on eight as a reasonable number, and aligned their start points with the 1st day of eight different months. But they could have had 6 or 9 or 10, and could have had them start on any date.