D
Have you used them John. You will still have a water/air point for the rot to take hold and if you are too efficient, you make a swimming pool for your post!
Unfortunately, the construction industry is littered with 'good ideas'. The prinicple of these things is fine, in that they are trying to eradicate the air at the point where the soil is light, friable, and air-entrained.
In practical terms, they're probably pretty b***** useless (sorry Jon) as the polythene will never be an airtight fit around the pole, just as in the same way, you need to use conductive grease on an LGB sliding fishplate.
I remember years ago when Catnic introduced a metal door frame for internal doors, in knocked down form - a hopeless, impractical disaster
Treated timber is your best bet for fence posts (if you can't get larch - which, after all, is a hardwood, being one of the three deciduous conifers (Taxodium and Metasequioa being the other two before anyone asks
)
D