We always do one park in the AM and if we decide to go back at night, it is either Epcot or MK.
We go in the summer and it is hot, so a mid day break is required. Sometimes, the kids are too tired to do anything but eat dinner and hang out at the pool or the room.
Since we were most often at the CR, and our kids were older the last few trips, DH and I would head to the MK for a stroll, ice cream, and to hit a few rides like Pirates and Haunted Mansion.
Having the ability to park hop allowed me to book ADR's at whatever restaurants I wanted for dinner without having to worry about what we were doing that day ahead of time. We usually plan our visits to the parks the morning after a park has had night EMH. Those are not always out when we were booking reservations so I didn't want to lock myself into any one park on any given day.
Last year, we were staying at VWL and had to return our rental car at the Dolphin. We did that, took a boat to Epcot, and because we had park hoppers, we were able to just walk through Epcot, get a slushie, and then head to catch a bus back to VWL. Had we not had the hopper, it would not have been as easy.
Now th