Can you pay cash at a DVC resort instead of using points? Example - Saving points for next summer and pay cash for short 2 or 3 day stay in between. Or are cash reservations to fill in for points that you don't have for an upcoming vacation?
Another option is to have points transferred into your account from another member. If you can get them for $10 per point it might be cheaper than a cash reservation.
If the cash reservation you desire isn't available through MS and you get it through Disney Reservation Center, you won't get all your member perks. Like free internet, the ability to pool hop, etc. You can still use your member card to get shopping and dining discounts, but not things that require you to be staying on points at a DVC resort.