I think what you are missing is that the band is more than just NFC (I think that is near field communication) this is what phones and credit cards and the new room keys use, you need to tap the sensor for the chip data to be read. Many credit cards have these chips and they can be used at many stores w/o the need to swipe the card.
I assume (based on a thread on this site) that these bands will broadcast a signal (which is why they needed FCC approval for their use). To me this means it will be a bit easier to know where you are.
Folks keep mentioning that there is no 'GPS' involved, but they seem to forget that the new Disney app is a part of 'next GEN' as well, so if you are using the APP to check wait times, make reservations, check your FP schedule...well, Disney can use both GPS and triangulation to get a rather accurate idea of where you are (triangulation measures the signal strength of three cell towers to determine how far you are from each tower as a way to pinpoint approximently where your phone is located).
Now it is possible that they need the bands to broadcast in order to use this technology for ways to make some of the attractions more personal, but then why are they giving away free WI-FI and tieing so many aspects into their new APP? Also, can their network triangulate the location of your magic band since it broadcast a signal?
I imagine these other parks folks mention that use wrist bands for this stuff, well I bet they are just NFC type chips rather than the ones that Disney is rolling out.