I've been reading a lot on Propylene Glycol (mainly since the juice I use has a high ratio of PG/VG) and I've learned a lot. Below is my final hypothesis based on the facts I have found.
First, there are some distinctions that need to be made between the use of Propylene Glycol (PG) in cosmetic products (as moisturizers have been used as an example to refute the dehydration theory), and that used in our e-liquid. For cosmetic products, studies have concluded that PG is considered safe only in concentrations up to 50%. What we vape is Propylene Glycol USP, considered as food safe, and is concentrated at just over 95%. PG in cosmetics comprises generally only 5% of the product, but comprises a much larger percentage in the e-juice depending on the ratio. While it can pull moisture from the air, air contact is extremely limited, only occurring when a container is opened. Even if it were, the humidity levels would need to be very high. This plays a factor in the grand scheme of things. Also, from what I have been able to find for e-liquid composition analysis, water content is very low. To say that PG is hydrated and thus needs no more moisture is false.
It has been stated, and is true, that PG is used in moisturizers due to its ability to hold moisture. PG is also a surfactant and thus breaks the surface tension of water. This allows the water to move more freely into your pores carrying any intended products along with it (including the PG). The problem is that as the water evaporates, PG seeks to replace it in order to maintain its balance, and since it has been able to get into the pores, its most abundant source of water available to it is in the dermis layers of the skin. So while moisture may be maintained on the top layer of skin, you do so at the sake of drying out your new skin. This is why you sometimes end up having to repeat putting moisturizer on if you are already dried out. Also, PG tends to leave a residue, and since it has been able to get into the poors, it does not wash away easily.
Of course, this also means that PG is an excellent vehicle for being able to deliver nicotine. As you breath in the vapor from the e-cig, some of that is going to cling to the inside of your mouth, long before any gets into your lungs and mixes with any water vapor that is exhaled. As it does, once again, PG goes to work by breaking the surface tension of the water, allowing it, and the nicotine to get into your poors. The more you vape, the more PG that gets into your poors, and the more water it ends up absorbing. Now, what do you think happens when the water on the surface of skin in your mouth evaporates? The PG draws it from beneath the skin. Eventually, as the amount of PG on the skin significantly increases, it draws even more moisture from you to the point that as more water evaporates on the surface of the skin, there is less water for it to draw upon from within. And as it has been shown that PG does not wash away easily, the build up will be more during periods of heavy vaping.
And that is my hypothesis based on the research I have done. I have sufficiently proven to myself that it does dry out the skin in your mouth given enough time, and under the right (or wrong depending on how you look at it) conditions. I also believe it to be practically impossible for it to cause one to become moderately or severely dehydrated and require medical attention, and any mild dehydration symptoms seem to only be localized to the mouth and not likely to affect the body as a whole. There would be several factors that would affect how fast, or at all, the skin in the mouth drys out, including the humidity level, water content of the individual, the amount of PG in the e-liquid, and how often you vape.