Funny all these people defending the principle that a data allocation should expire after a month. It's not milk. It's transmitting electrons and radio waves.
There is no intrinsic reason why 1GB of data that I buy today cannot be used tomorrow or three months from now, or why it would be difficult for a network to do that, especially considering they've been sitting on your money in the interim.
The only reason to structure it the way they do is greed and the continued confidence that the consumer will buy into their artificial paradigm of "expiring" data. A good demonstration of this attitude is how they will always use your newest bundle first.
I'm all for capitalism - they would be remiss if they didn't exploit the opportunities, but they've dipped their hands too deeply in the cookie jar and blown the illusion that expiring data in a few months is a necessary thing. If they want recurring revenue they'd be better off selling pure access time with a speed throttle, not access time plus a quantity limit.