Read the links. The units have venting systems; usefull for moisture and fresh air.
Yow! That's an uncharacteristically snooty reply...
I would also be concerned about severe condensation no matter how big the vent holes are - and if they really were big enough, they'll let too much wind/rain anyway. Any experiences I have had in non-porous enclosures have been moist to the point of dangerous. The exception is the more extreme alpine conditions most of these botha bags seemed to be designed for. For close off the trail shelter, I think you might do a lot better with a lightweight inexpensive polytarp as described (for example) in
The Ray-Way Tarp Book...
<img src="/images/graemlins/smile.gif" alt="" />