If someone is willing to experience everything any of the parks has to offer, they would all require at least a full day. But in practice, that is rarely the case. Especially for frequent guests.
I guess I'm the exception to the rule, because I'm a seasonal resident and EXTREME frequent guest, and my favorite type of park visits are still the open to close, rope drop to shutdown, full-day, "complete" park visits!
I like to completely immerse myself in the world of each park and get the "complete" experience. I view the parks as "works of art" so to me skipping large amounts of them feels like watching half of a movie or listening to the first half of Sgt. Pepper by the Beatles but not the second half (side 2, in the old days!)
I still do some partial-day type of visits, such as park hops after a full day at AK, or 6 PM-close type visits when I don't have the whole day free, but I find in those instances I gravitate towards the same favorites every time and end up skipping things that I actually like, but don't have time for.
What I like about full day visits is I get to experience the entire breath and scope of a park, including attractions that I like that would get left out in shorter visits, as well as savoring the "little design details" that make the parks so special.
The bottom line for me is, if I start at a park at rope drop and have the whole day free, there is enough at all 4 parks to keep me occupied for a whole day without repeating myself or getting bored! I guess I should consider myself lucky that I like nearly everything the 4 parks have to offer!
When people say "you can do DAK in half a day" it's understood that you will be skipping things. The implication is that certain things are okay to skip.
I think that's fine and good if they explain that it is their OPINION. What I disagree with is when park "newbies" are asking how much time to allow for each park, and people tell them they can finish AK in a half-day, implying, as you say, that certain things are OK to skip.
Well, maybe the person asking that question is an animal lover like me, who could EASILY spend a whole day just enjoying the self-guided "look at the animal" attractions!
I would recommend for EVERY person who has never been to WDW before to allow AT LEAST a full day for each park and try to experience as much as they can at a pace they are comfortable with, and also ideally allow a 5th or 6th day to do anything they may have missed on those full days.
Because it's best to find out for yourself what you like or what you may dislike, and also, a newcomer won't be familiar with the park layouts or "touring tricks" to maximize time that "veterans" know.