The article in the WSJ on this raised a valid point. Polls have rated the empty middle seat as more important than legroom--and fewer seats means less chance at an empty middle.

This also makes the exit row the worst seat on the MD-80--it loses its legroom advantage and still has the wall between seats (I'd take half an inch of width over two of legroom anyday--a big part of why I like US and AC is that they fly Airbuses).