I found your comment interesting. So I took your advice, and Googled it.
This supports the argument that in low dew points glycerin will suck moisture out of the cortex of your hair.
The second result is the page on the same site that the first result took me to.
The third result is a short conversation on CurlTalk, which then links to the article above.
The fourth result
again yields the information that when there is little water in the air, humectants will pull it out of hair. So, not a myth.
The fifth result
takes me to information about glycerin being counterproductive in high dew points because it can draw too much moisture. So not relevant to this discussion.
The sixth result
states that "If you can avoid most of them (which can be difficult) you’ll do a lot to help keep moisture in your hair." It also links to this article
on NaturallyCurly.com, and cites this quote:
"In extremely low-humidity conditions, such as a cold, dry winter air, there is no appreciable amount of water in the air for the humectant to attract to the surface of the hair. In this particular type of climate, the best one can hope for with most traditional humectants is for them to prevent evaporation of water from the hair into the environment. Also, under these circumstances, there is some risk of the humectant actually removing moisture from the cortex of the hair itself, creating the problem it was intended to prevent."
So, again, this does not support your contention that humectant use is irrelevant in low dew points. I could go on, but there's a definite trend here.