I can't pass up a good debate!
Substrate usually shifts to where the smallest particles end up on the bottom. Small stuff just falls through a screen. If it's water space you want on the bottom, use a plenum but place a screen halfway up so some organism doesn't mess it up. Damsels are often guilty of digging holes in the substrate. Opening up a mature plenum to the main water results in an algal bloom. A damsel did it to me in my second plenum.
In the end after trying lots of substrate methods, it sems to me that the use of a mixture or separate layers seems to work just the same. and forget the plenum. Sand on the bottom works just as well. It just seems that fine stuff on top is hard to keep looking nice. A sand sifting star helps but also eats the faunal population.
Any time a "sand blowing" goby is added the whole substrate issue becomes a mess anyway. The goby disturbs the bacterial zones to some extent but then they re-establish themselves.
The one thing I definitely believe in is bacteria and a faunal population in the sand. You can use fine sand and get away with an inch or two of depth, but if just "puca" is used I would recommend 5 inches.
My 2 cents