A lot more people by one day passes than you would think. I used to work at MK (as recently as a few months ago) and at lunch or after work in the tunnels I would bump into a lot of the ticket sellers and I got to become good friends with a lot of them. Anytime I would ask what the most common ticket they sell is (naturally being curious as a fan), the answer I always got was the one-day pass.It really isn't.
One of the UK parks (Thorpe Park) is $70 per head entrance fee, and you're completely not getting anything near the level of a Disney park. I never pay $70 (£45) per person because there is ALWAYS a 2 for 1 offer, I don't know many people that would go there and pay full prices.
Same as at WDW, how many people actually go buy a 1 park 1 day ticket? Not as many as those that have multi day tickets surely!?
Seems stupid, but people pay it. Sometimes it's corporations if they are there for a convention, maybe locals who don't go to the parks often, whatever. Over at Universal it's similar, but they sell the one day, two-park passes the most last I knew.