Well that was my thoughts , but just noticed that there is another thread just started on the same subject .... hope this is not going to become another trend along with charges for trailers , colour of your car ....etc etc !
I'm not sure but I think they cause condensation. I used one in my tent over the SIG in November and the tent was coverered in condensation on the inside roof lining. I asked about it on here thinking it was the tent but I'm sure someone suggested it may have been the breathable ground sheet. I cant remember so may be wrong.
They arent for COVERING the SIG inside. The idea is for them to allow the grass to breathe (admitedly in awnings etc only)- which its never going to do when covered by a SIG whatever 'footprint' is used.
Ive taken to using mine under my SIG, for no other reason than if the site 'demands' one, im using one - wether it does a blind bit of good or not - which it wont! I also feel itll let water pass away from the tent better than a sheet of plastic of any sort.
My way of thinking, only.