IMO, NextGen looks to be Disney's Big Brother system. They are cleverly trying to disguise it and sell it as something that is going to be better for guests. Better in the sense that you won't have to carry cash, purses, etc. You'll just swipe your bracelet (or whatever it is) and your meal is deducted, charged and you didn't have to fumble with anything to get your stuff. They are also putting this system to use with the fastpass system. So, not only are the Disney moneymen tracking your every purchase, but they are also tracking an infinite amount of other things as well. This NextGen system is going to track when you enter and exit a park, your location (via purchases or swipes to get on rides), how much you are eating/drinking and how much you're spending on doing so.
My guess is, they're going to use this data to gain exact figures for demographics. If Disney can say that the Jones family was in the park on X date from time A to time B, rode this many rides, bought food here and there, spent this much on it, then left and went to Y location and do this for everyday of their trip, they're going to gain knowledge on their customers. They will then also be able to track all of this based of where you are staying (I'm guessing if on property) and they can then track what you do when you're not in the parks either (I believe I saw a rumbling that they are looking at stuff for buses?). All in all, they'll be able to track your every move without you ever knowing it.
I honestly think of this as Rollercoaster Tycoon where you hover over a person and it tells you what they spent and if they're happy or not and you read what they are thinking. NextGen is a little short of this, but it's not far off.