Why you shouldn't stack speakers

Not to pick on anyone in particular but I recall reading some guys like stacking speakers and I remember reading elsewhere that this is not good for accurate sound reproduction.  Just this week I found a discussion with measurements on why this is a bad idea.

http://www.audiocircle.com/index.php?topic=124751.0

Post#15 and thereafter.

Sounds like valid science but it doesn't comport with my experience: stacked OLA/NLAs sound much better than a single pair. Many people have been stacking Advents for 40 years and these is general agreement about the synergistic results. Perhaps there are some unique aspects of Advents that change the physics involved.