You are correct, and we did something similar a few years ago, had a quick weekend trip and bought party tickets instead of park tickets. We were there from 4pm to 12am and the party tickets were cheaper than if we had purchased 1-day MK tickets so it was good value for us. Of course, that's not saying much, given how expensive a 1-day ticket is.
It's a different story for the many visitors on vacation for the week. If you have 7-day tickets, that's an average $50+ per day, it's a lot cheaper to add another park day for only $10 than to add party tickets for $72-$95. Similarly, knocking a day off your multi-day ticket is not going to save as much money as it will cost to add party tickets. I can see how some people might not see the value of adding party tickets on top of their multi-day tickets.