It makes perfect sense to Disney. After the 4th day, each additional park day costs only three dollars. Park hopping costs $50. They'd never sell another park hopper if you could pay 3 bucks to hop for one day, or 6 bucks to hop for 2 days, etc.
The important thing to realize, as was mentioned earlier, is that you aren't buying "admissions" to a park, you are buying DAYS of park admission. If a base ticket has 2 days on it, both DAYS can't be used in one single day.
Now, to really complicate things, it's the opposite for the Water Park Fun & More option. If you buy a 5-day ticket, you get 5 WPF&M "admissions." You could go to Blizzard Beach in the morning, Typhoon Lagoon in the afternoon and DisneyQuest at night, and you will use 3 WPF&M admissions on that single day.