It's not decay it's horizontal dispersion. The quasi ribbon used in the MMG is actually part of the same diaphragm as the rest of the speaker. The horizontal dispersion is not very broad. It is better the further the listener is from the speakers. Unfortunately the smaller rooms they (MMG's) usually get placed in don't allow for "far field" listening. Get 10 feet away from some raised MMG's and the sweet spot is a fairly decent width.

IMO MMG's sound good even out of the sweet spot. They're just better in it.

The rear wave adds to the openness and depth of the sound. It doesn't help with the size of the sweet spot. The wall behind them (any dipolar) should be a mix of absorbing, reflective and dispersive surfaces.