The ticket you buy for the day, if it's for just one day at the MK, then it has an advertised time of 8 AM to 6 PM. You can decide if that's worth it to you. Or, buy a one day ticket for a day the park is open 8 AM to 9 PM on some other day of the week.
If you buy tickets for a bunch of days then those tickets can be applied to any day whether the park closes at 6 PM or 9 PM. You'd probably want to chose the day it closes at 9 PM.
If you want to go to the MK from 4 PM to 12 AM on a Party Day, then you buy a Party Ticket for that.
People who want to go to the park for 8 hours and enjoy the Party, can forego going earlier and spending for the 'double ticket.'
If, however, you want to go the MK from 8 AM to 12 AM on a Party Day, then, yes, you need to buy a day-ticket and a party ticket. If you have an annual pass, then you just buy the evening Party Ticket and still go all day (annual pass holders wind up paying a lot less for each day at any park than those who buy day-tickets, so, the total cost for them needs to account for that discount).
Also note, assuming they're still doing this, that a guest does have the option of paying for the extra magical breakfast deal which gets you in an hour or two before the park opens, plus breakfast, for $80/ea. It's just another option if you want the entire day at the MK.
They could really sell it by the hour and everything you said would be exactly right. It's their prerogative to do that. You could explain everything you just did, which is factual, and say, "It's up to you to decide if you want to pay for the MK by the hour @ $20/hr/ea."
The interesting thing, which no one has really thought about or mentioned, yet: "By the hour" (or even by the minute @ 33¢/min/ea would be a better deal for the guest. Instead of buying a day ticket for some arbitrary amount of time which changes based on parties/nights/etc., you're just paying for the time you use. If you want to stay the full day then you pay the full day's price. If you only want to come in in the evening at 7PM and stay to 10PM then you only pay for 3hrs. It would encourage people to leave who aren't so keen on being there and are just "hanging out"/"looking for something to do". It would stop people from trying to get their "full value" of the ticket and they could just use what they want to use.
It'd probably increase overall capacity of the park because you'd have more guests/day which means more individual wallets/day.
I know people like the "all-you-can-eat" style of purchasing things but it really doesn't work in your favor. I also think that paying by the min would be a far more honest approach than the current system.
Looking at the current system:
Let's say you purchased a day ticket. This is $125 but the number of hours it covers varies pretty greatly (though, as noted, they're posted so it's your choice). It may be a short day of 9AM-6PM (9hrs) or a particularly long day of 8AM-midnight (16hrs). That's a pretty good difference.
Now consider a party day and you're going during the day and night: Your day ticket covers you from 9AM-7PM (10hrs) and your party ticket covers you from 4PM-midnight (8hrs). You're not getting 18hrs out of the park, though. You're buying 3 overlapping hours which, as noted above, you could certainly say, "Well, you knew that going in..," still, it's the same game that other companies play to sell you more than you really want/need.
Think how you buy 3GB, 6GB, 10GB, etc from your mobile carrier - it's all about you buying more than you need. You'd be better off just paying some ¢/MB amount. Then you're only paying for what you use and not ending up buying waste, which is what the business model for most of these companies are. If you're a heavy user then you're going to pay more - but that makes complete sense. If you're grandma and you're only doing some instant messaging, then you'd pay WAY less - that too makes complete sense.
The current system of selling the park multiple times a day doesn't make a lot of sense and isn't a good guest experience.