I have only cruised once with the app and a medallion, and I agree both have advantages, the app makes it easy to see your on board spend, and it can be handy to check things if you forget your patter, and the medallion must be great for families to check the whereabouts of everyone, and it's useful for wheelchair pushers to be able to take their time reversing into the cabin, especially as Princess has not yet joined the 21st century and installed automatic doors for accessible cabins.
However on our next cruise I am definitely not using the Dine My Way MDR booking, that is just too silly, we will turn up when we fancy at the MDR of our choice and request a sharing table, and hope we get interesting table companions, like we used to in the good old Anytime dining days.