Diffusers: under what circumstances might they help when placed behind speakers?

OP:  I think you hit it on the head.  Generally speaking, acoustic treatments make things better in the dimension you add them, so if you want depth, diffusion between and behind the speakers is good, as is behind the listening location. 

Also, don't underestimate the value of absorbers on the ceiling between listener and speakers.

A diffusor placed centrally and behind the speakers is very often done in high end showrooms and private listening rooms.  Adds clarity without deadening the mid-treble range.