I think that it was TWA who had More Space in Economy - they were acquired by American in 2001 and AA did not embrace the idea after the merger.
At the time TWA was doing poorly and needed something to differentiate themselves. However the airline continued to have a yield problem. This may have been caused by other factors - limited route network, poor reputation, etc. It didn't save the airline.
The yield per square metre of floorspace is generally (hopefully) higher in economy + compared with economy hence its popularity. That said if demand for the product is low on a particular day so can get good deals.
If you travel DL (and I would imagine other airlines) you will see the number of spare F & Y seats and the fee for upgrade (with an eligable fare class). In other words, if you want a free seat next to you don't travel F. Mind you load factors are so high these days you don't often get an empty seat next to you in Y very often.
Airlines line BA have a strict upgrade policy (in my experience it has to be authorised by by a supervisor on the ground and the captain in the air)
Last edited by Peter47; 11th Mar 2012 at 19:01.