With FP+, if someone gets a 7:50 PM time for Soarin', they are more likely to be there. It's an appointment they scheduled months in advance. They go to DAK in the morning (maybe) and head over to Epcot for dinner. If someone doesn't understand FP+, they have had months to stare at the thing on their home computing devices, with WDW sending them email prompts that "it's time to book your FastPass Plus experiences". Even if they don't fully comprehend it, they are going to click buttons and make selections. People have been trained to do this. (I've started several polls only to have people effectively write, "I didn't understand the poll question but I made a selection anyway" or "I have no experience with the first 3 options but I've done the 4th option so I picked that one."
) Once they make their selections, they've made an appointment and are more likely to keep it. FP+ should help WDW more accurately predict crowd levels which should help with
efficiency if, as you allude, people don't end up confused by it. Despite WDW's best attempts, there still will be those stopping at MK Guest Services asking, "I've got a time for Expedition Everest. I can't find it on the park map. Where is it?"