During our first trip (May 2011), we did add on the park hopper option at the park because of unexpected changes in our itinerary. We went to Animal Kingdom in the morning, and wanted to go to Hollywood Studios that night. We only used the park hopper that one time during the trip.
During our trip last month (Dec. 2012), we didn't buy park hopper and didn't miss it. We're not planning to add it on for our upcoming trip (April 2013) either.
We tend to get to the parks later in the morning, and stay as late as possible. We're not the type to leave the park for a break in the middle of the day.