DCL does provide other things that other lines don't, namely the character photo ops in the terminal.
I agree that a line adds to the excitement. Disney is about telling stories..
and there is nothing like starting a story then a good build-up of anticipation--I think the big line adds that feel to it. I think if I was sitting in a chair, and then it was time for my "row" to board, it just wouldn't feel the same....
I also think the the huge open terminal, with the high ceiling and marble look adds to the the feeling, To me, huge rows of chairs would make it look like a bus terminal.
I think it makes it easier on families to make it more open, so people can come and go.. let the kids run around, see the model and characters...if there were chairs it would be harder for kids and their family to do that.. stepping over people in chairs..
I usually walk around myself.. Take pictures on the landing, get a coffee, look at the model and take pics with the character, Usually get in line about 20 mins or so to boarding and still get on the ship by 12:30ish. DCL has the best check-in/embarking and disembarking of any line I have been on--so I never feel pressured to have to wait in the line for hours, because it always moves so fast anyway. And I think it's great how they have the buffet on two levels, so don't have to wait in a long buffet line once you get one.
DCL is just the best. It's like they say, once you go DCL you never go back!