One reason i like the paper tickets is because you are not allowed to take your phone on certain rides. So once you lock your cell phone in a locker, you won't be able to scan your ticket to get it out. There are attendants with locker tickets that you can use, but it can get chaotic at the lockers and it could take up more time.
Another reason I like the paper tickets is because the lines (entry gates and Hogwarts Express) seem to move faster with paper tickets as opposed to one person showing their entire party's tickets on one phone. Scanning multiple tickets on one phone just seems to clog the line.
Yes, there is a guest services location in Citywalk. Coming from the parking deck, I believe it is on the left just before you get to Red Oven Pizza. There are also guest services locations for both parks off to the right just before the entry gates.