Much of what you say is true Dave, but consider all that gets added to plastic: hi lite powders, glitters, glass beads....it all contains air, but certainly does not mean that it doesn't hold any moisture too. It doesn't matter how careful one is in handling the plastic, air and moisture gets introduced in a whole host of ways at the user end. I honestly feel that most of the sellers of plastic do a great job of providing a product that has been made in the most stringent of conditions but beyond that its the user that generates most of the issues that get brought up and usually its done inadvertently or thru haste.
Plastic in the bladder packs allow air in no matter how careful a person is. Simply heating cold plastic will generate moisture, even in raw plastic from the bladder packs. If people find that spending the time or money to "de-gas" their plastic, by all means do it, but those same people need to understand that glass and plastic cooking vessels have porous surfaces that harbor both air and moisture in minute quantities and that the act of heating the plastic releases and expands those elements. I use all glass pyrex cups and keep them "kitchen" clean and dry. I also put each cup in the microwave for twenty seconds without an thing in them and give them a good wipe with paper towel before I even think of adding any plastic to the cup. All of my cooking is done in an un-heated garage but every single component that goes into a cup is kept in the house in a very wary utility room that's constantly de-humidified. While its cold out my cups are also kept inside. I mix my batches in the house when its cold and carry them to the garage to cook and I never, ever cook when its below 35 degrees in the garage and only then after I run some empty cups thru the microwave for a minute and then give the inside a wipe down.
Like I said, if people want to de-gas that's great. My personal feelings are that it does nothing that some fore-thought won't take care of.