To the question, "who pays for a single day admission anyway?" I answer: many people, especially those who are casual visitors to the Magic Kingdom (usually the ones who call it "Disney World"), most of whom are first-timers.
I would also answer the question by saying "people like myself". I am a total Disney World junkie and would be happy vacationing there every year if I could. But last year, the wife wanted to do something different, since we went in '04 and had planned to go back this year. She thought it would be nice to visit Texas, so we did. That was to be our primary vacation for that year. However, we only took a week instead of the two weeks we had taken in Florida, and we were pretty much done midway through the week. Having an extra week of time and extra money (sightseeing in Texas is alot less $$ than theme parking it in Florida), we planned to do a little extended weekender in Florida. Visit my aunt and the beach and go to Downtown Disney and EPCOT, then head home. So we purchased one day tickets to EPCOT and went there and got to try out that new Soarin' ride.
So, to make along story short, people who make one day visits would include people such as myself who have to get in a quick "fix" until their next big trip. People who don't have the time or money to make a whole vacation out of it, but miss it enough just to go down for a few days, do a park and then head home. So like I said earlier, next year, we are going for 6 nights, but are getting the 10 day parkhopper passes with no expiration option, so that we will have 4 days left to use on any future trips, getting us in for less than $40 a day, prepaid. Say we want to go down there for a week or so and do Sea World and Bush Gardens. We can throw on a day at Magic Kingdom or EPCOT at no extra charge, as it is already prepaid.