I work in an environment where I can't use a cell phone. So out of any given billing period I have extended periods of time where my data package is no good to me as I can't use it. I can't carry unused data over (which is another good idea) so I would like to be billed according to how much data I actually use. Some months it might be a lot while others hardly any. Just deciding on one package doesn't work for me as from month to month my usage varies greatly.