The reason they make you request to stay on a wait list once you get within 30 days of arrival is so you don't get blind-sided and have points go into the holding account.
If you have a back up points reservation (as many of us do) and the reservation you want is less points than the back up (that will be canceled), the "extra" points would go into the holding account. Holding points cannot be banked, expire at the end of the use year and may only be used to make a reservation that will begin no more than 60 days from the day you make the reservation. It's often very difficult to use holding account points - so MS doesn't want anyone to be unhappily surprised. This is even more important now that they don't call you before they make the reservation when the wait list works.