Tom C.
I fully understand your logic.
While at the same time, you want to get air to the roots - you also want to curtail moisture evaporation. Even with the plastic moisture barrier, sufficient air does get to the root system - I guess it is all a mater of priorities - keeping the roots moist trumps air exposure.
The porous nature of the Perlite and air gaps between the Microbark will provide sufficient air exposure for the roots - that is why I never recommend people putting sand in their container mix.
Raybo
