A problem is that AA doesn't enforce MCE seating. A month ago I was on a transcon flight on a 738. No EXP upgrades (of course) and I was seating in an exit row with an empty middle seat.

But right after doors closed - a guy moved up from the back and took the middle seat. I noticed his boarding pass and it said BASIC ECONOMY and his was in the last row of seats.

Now the FA's came around and looked at the seat maps during food/beverage service. EXPs get offered a free food item and free drink, although MCE get free drinks now. This FA could have questioned the person in the middle seat but they did not.

On UA, the FA's check at the beginning of the flight and even throughout. So if someone move to a MCE (E+) seat FA would ask to see their boarding pass and then ask for payment. If no payment they must move back.

If AA is not going to enforce MCE - they why offer it as an elite upgrade or for purchase.