While i agree that for pretty much the rest of the dungeons it would be cool if it was just continuous space in coil itself it actually makes sense it isn't since from turn to turn you are going down an elevator, deeper and deeper into the structure as you go.
Well, while I actually don't know why they make you go through those portals, it's not so that they can split the dungeon into smaller instances, as that just doesn't happen, for example you can attack the mobs upstairs in AK before you actually go upstairs. Also when you die, you go back to start.
