You pointed out the beauty of AT&T's plan. If you don't use the data on month 1, you can use it the following month.
Please see my other comment for detail of my particular situation.
You think it is a beauty, I tend to think it is a backhanded way to provide it without it being of much benefit. It is only beneficial if you alternate from months without overages to months with overages. If you don't, there is no benefit to it at all. Seeing as I have never had an overage, I would be in the camp that it does not provide a benefit.