This seems to miss the general point. There's still middle seats on MD80s (although your theoretical odds of ending up in one are 20% versus 33%), and AA is (slowly) phasing out the MD-80s in favor of 3/3 737s in any case.

No AA planes have E+, though, so you're comparing an advantage on full flights that you don't book reasonably far in advance on a specific type of aircraft to a benefit that UA has nearly fleet-wide, even on some RJs.