I recently had nearly all my MS points deleted off of my account, over 6000+. According to support, they expired.
Some of these points where promotional points earned from Bing rewards, but the bulk where from christmas gifts of point cards from Christmas 2011.
I contacted support, and was told sorry, nothing they could, once points expire they are gone forever. When I pointed out most of these points where points purchased with a gift card, I was basically told, "prove it". Without the original gift card, they claimed there was no way for them to know how I recieved the points, so they could not help.
As far as I am concerned, the points where stolen from my account by Xbox. I will NOT be purchasing any additional points in the future and will not be renewing my account. Again, I understand perfectly that promotional points expire and have no problem with that. Points purchased with real world money are NOT suppose to expire.
So beware, keep your cards, if your points are "some how" deleted, you may have to PROVE you really purchased your points or your SOOL.