What's special about Hvar or Zadar?

Asking the real questions. You’re not wrong. There’s a great deal of similarity in the region. The towns are all basically Venetian fortified port towns, and the landscape is consistently “rugged beauty” with crystal clear waters. Like, everywhere was beautiful and amazing. Does that make a place like Hvar redundant? Not exactly. It’s like looking at a beautiful painting and saying you don’t need to see another one. But it’s certainly worth weighing into your itinerary planning. Like, there’s not much sense in spending a night apiece in Hvar, Brac, and Korcula versus all three nights in one place, but staying or visiting one of those islands is a different experience and “worth it.”

So, would you be missing out? I think it depends on the time. Split and Dubrovnik are busy cities. If all you had were a few nights between them then you would not feel like you were missing out. The day trip and beach options there are plenty satisfying (Lokrum is as fine a place to hike or swim as anywhere else.) If you’re just thinking about day trips, you can play this by ear for the most part. I pulled out of a few excursions/day trips for basically the reason you describe. “It’s awesome here. I’m just going to hang here.”

Zadar’s old town is very small. Except to sit a listen to the sea organ, there isn’t much else. There is an old church with absolutely nothing to see inside. We stop there during our yearly trip to see a cousin and his wife. If not for them we would skip it.
On the other hand, we would never skip stopping in Split and Dubrovnik. In fact, we’ve been to Split 4x and still haven’t gotten to the Maritime Museum.
We’ve never made it to Hvar.

How many nights will you be in Split? We spent 4 nights in Split, and we visited Hvar for a day. It was one of the highlights of our trip.

Hvar's Old Town is different from the Old towns in Dubrovnik and Split. We followed a zig zag path, lined with pine trees and cacti, up the hill to the fortress. Along the way, there are benches where you can rest and enjoy the spectacular views of Hvar Town and the harbor. From the fortress, you can see more dramatic scenery including some small islands.

The promenade/coastal path was a highlight for us. The scenery is gorgeous! The path leads past a cactus garden, a children's playground, and green pine trees, so there is lots of greenery contrasting with the blue skies and blue water. There are small bays where people swim and sunbathe, and where you can rent loungers. And treat yourself to drinks at a cafe overlooking such gorgeous scenery. What could be better than this!