1. Because park hoppers are priced without consideration to the number of days, they become less expensive (per day), the longer you stay.
2. They add significant flexibility to change plans. Finish a park earlier than expect, park hop. Decide you want to spend 1 1/2 days in different parks, can plan around that.
So let's break this down into three categories:
Park hoppers "necessary":
Staying under 4 days, but still want to squeeze in all 4 parks. Obviously, can't do it without parkhoppers.
Park hoppers probably not worthwhile:
A "medium" length trip of 4-6 days with very firm planning. Especially if you do the parks in 1 stretch. For example, young families who may do morning, but generally don't do nights in park. Or those who show up at noon and then just stick around.
Park hoppers probably valuable:
-Long trips. If staying 7+ days. For a 7 day ticket, it works out to $12 per day. Not much at all, and I'd look at it as "insurance." Even if you aren't sure whether you'll use it... it's extra insurance. Say you get a Rise of the Resistance late boarding group. You're at DHS before 9am, but you won't be riding RoTR until 6pm or later. You now have the freedom to hop over to Epcot for the first part of the day. Or maybe you're staying at a Magic Kingdom resort.. you finish Animal Kingdom earlier than expected and not interested in returning for the night. Nice to have the flexibility to hop over to MK if even for an hour at night.
-Those who do Rope Drop + Night entertainment. Why be obligated to return to the same park as you started the day, especially as different parks have different night time flavors, different closing times. Maybe you're a night owl, so you want to go to the parks that are open latest, regardless of how you spent your morning. Maybe you love Epcot's International Gateway at night, with all the dining choices.
-Those who utilize a lot of EMH: The good side of EMH is lower crowds during EMH. The downside is those parks have larger crowds the rest of the day. So park hopper lets you split your day between EMH park and a low crowd park.
-Guests of deluxe Monorail hotels and guests of deluxe Epcot area hotels. Why? Because you're hotel room in 5-20 minutes from being inside a park. Come back to the Contemporary after spending most of the day at Epcot. Your family isn't in the mood to go back to Epcot, but everyone would love to do a couple of Magic Kingdom rides one more time. It would suck to be right across the street from MK but not be able to go.
So the guests for whom the parkhopper isn't necessary but a great value:
Those staying on a longer trip, at a MK or Epcot deluxe hotel, who are Rope Drop + Night type guests.