Phones would work for all guest use applications at WDW. The problem is that Disney wanted the device wearable in order to increase operational efficiency. This way there is no waiting for guests to get out cards or phones at main entrances, Fastpass entry points, or registers since they are already out and in your wrist. Additionally phones would not have allowed for the automatic linking of photo pass images, interactive elements around the resort, and data collection since not all users would have this capability and many utilize enhanced security/privacy features that would prevent it. Lastly, even in 2016 not everyone has a cell phone and certainly not a phone capable of what Disney wanted. And while most families probably do have a phone capable, they do not have one for every member of their party.