We have always gotten the park hopper on our tickets.
We find MK is fun but only early in the morning, or late in the evening (on occasion). It just gets a bit congested for our taste during the lunchtime.
We prefer one park, say MK, early, then go relax maybe somewhere (even having lunch at a MK or Epcot resort in the counter service area, or back at the hotel) then usually enjoy Epcot or DHS in the evenings.
Going from one park to another is usually quite easy, and we like to mix it up. We don't tend to stay all day anyway (DS has asthma so during hot times this isn't good for him, and he has ADHD as well so changing it up helps as he doesn't take meds on vacations plus we get a GAC to help him wait in a cooler spot during hot times or else he gets attacks).
To us it is a no brainer, but to some it's more fun to stay at one park all day. Also MK doesn't sell alcohol, and we (well, I) like a glass of wine or a cold beer in the evening when it's hot out - although we've gone to the Poly once with a friend so she and I could have some wine and the kids could relax on the beach area and have a late lunch and get away from the crowds.
Like the others said, you can add it later if you want it!