Simple. The 757 will be going away long before the ER's do. The ER's in many instances will be here for another 20+ years. Therefore you want to pull it up. You will not be able to pull the 739 up to the WB band, but most will not agree that it should be in the NB band. It is effectively the CAL Wide, LN, SN scheme. It also allows for the 787 to pay the same as the 767(wide) and 777 if you use three bands.

The middle band allows for the 321, 739, 757, 753 to all be in that middle band while keeping the predominantly international jets separate. It leaves some room for negotiation. If you wanted make it four bands, but then all you are really doing is banding the nb jets and keeping the 777 and 744 separate.

For you blood pressure, there is a lot of traction on banding. It just needs to be done correctly, that is all.