Last time I had to buy a battery pack for the photo center, the battery was reasonable. Maybe $15 with a free usb cable. Then went to buy iphone cable... $20. yikes. Had no choice, as we travel in large groups, 10-15. We split up after lunch, and rely on phones to communicate everything that is going on.
Personally, I like the MDE app. I love getting fastpasses all day. I love preordering my counter service food. I love knowing where the resort buses are, knowing "posted" wait times, and everything else, like reservations and show times. You can use it to maximixe your trip.
But it doesnt replace MB or cards. It just can't. Augment? certainly. My kids like there independence. With a band, I can trust them to be gone all day, feed themselves, and get back safely. I leave my wallet behind all week. and the phone battery is no small thing. We all carry battery packs, cuz we are all on our phones while in line... mostly refreshing for fast passes. lol. So we will all need to have cards., but can room charges be linked to cards?