We wouldn't think of going without a hopper.
Hoppers let us spend mornings in our "morning parks" - usually MK or AK, then spend evenings in Epcot - where there are lots of restaurants and we can spend the evening watching shows and shopping through the World Showcase.
Hoppers let us walk into the MK for Early Entry, and LEAVE two hours later when the park is crowded - allowing us to get a lot done in those first two hours - and avoid the crowds for the rest of the day by going over to DHS, Epcot or Animal Kingdom.
Hoppers give us flexibility to spend a day and a half in a park - and not waste half a day. After many trips, there are a lot of parks for us that need more than one full day, but less than two. And a lot of parks that don't even need a full day for us (Hollywood Studios - we get through Toy Story Mania and Star Tours and my son rides Tower of Terror, and we are done).
Hoppers allow us to change our minds if we discover a park is really crowded and we think we'd have better luck dodging crowds somewhere else.
As for wasting time - we generally break mid day anyway - so going back to a different park takes a much time as going back to the same park.
As for wasting money, we figure that the ability to dodge crowds and make optimal use of extra magic hours, plus being able to hop in for Illuminations or Spectro or Fantasmic add more value than the cost of adding hopping.
We also stay at the Boardwalk though, so being able to cut through Epcot on the way back from the MK and grab fastpasses for Soarin on the way to the pool, or run through the single rider line at Test Track is worth a lot.