"Can anyone debunk this myth or give credence to it with a pleasant experience sleeping in a wet synthetic sleeping bag some night?"
I don't know about debunking, or pleasant experiences, but I can share my one experience with a thoroughly damp synthetic bag. I and 2 friends did the Buckindy-Snowking Traverse in the Cascades about 20 years ago. It was a 5 day/4 night off trail affair and it rained every day. Everything quickly got soaked, including my Moonstone synthetic bag(rated at 32 degrees-supposedly), as we hadn't yet figured out trash compactor bags. Night time temperatures were in the low 40's and, while not the most comfortable I have ever been due to the damp feeling, I added a second layer of polypro, top and bottom, and slept warmly every night . Had the temperatures been maybe 5 or so degrees lower, it might have been a different story, I just don't know. But I can say that if you add a second layer of synthetic clothing, you should have no trouble down into the low 40's with a wet synthetic bag rated in the low 30's. One data point for you to consider.
Edited by Ouzel: Another point to consider is that your body heat will drive moisture out of the bag as the night wears on.