Floyd Toole did listening tests to determine if listeners preferred absorption, diffusion, or bare wall at the first sidewall reflection point.
Diffusion and bare wall were the most preferred....with absorption by far the least preferred for sidewall first reflection. Going off memory only like 14% preferred to absorb sidewall.
Why do so many people recommend absorbing sidewall first reflections? It has to do with pro-audio rules of thumb and advice given by acoustic product companies such as GIK.
Pro-audio wants as much direct sound as possible, to take the room out of the equation....basically a headphone type experience with speakers so their mixes with translate.
Hi-fi uses the room to enhance the experience. That's why we don't listen at a desk or shove our speakers up against the wall.
Having said that, very close sidewall or close behind our head reflections can be very destructive and smear the sound...as long as we have some space though, our genius brains can sort out the direct sound from the indirect sound and we get this really nice ambiance or reverb without the smearing.