I'm in the I don't care crowd, but certainly wouldn't complain about having my own seat.
I'm no engineer, but how is it even possible to put in seats?
On the 300 level I might buy it, because of the pitch, and the open area under your seat, for sodas, coats etc. 200 and 100 levels with the back have no open are underneath to put anything, and my knees are right up close to the back in front of me. When someone comes into the aisle upstairs, you can literally sit sideways and let them by, which is impossible to do downstairs.
I would love to see a mock-up drawing. Being someone who doesn't really know, I can't see how it's even possible.