The other 20% - good article
I have no data to support it, but it makes sense to me that entrapment, unconsciousness, mouth immersions would be the critical risks once you get beyond not having a life jacket.
Based on my limited experience (fortunately), entrapment is probably the critical hazard. There have been a couple of entrapment fatalities in my area since I started paddling. While everyone I paddle with does wear a PFD, I do know a few people who don't wear a helmet, so they could easily be knocked unconscious. I've also been stuck in a hole, and have bobbed down some long rapids, so I can also see mouth immersions being an issue.
Unless an inexperienced paddler does something really stupid, though, these are risks that effect a limited number of paddlers (whitewater and advance sea kayakers). I would also add hypothermia to the list since there have been a number of cold water paddling deaths in my area recently.
With so many other things to worry about, putting on a PFD seems like a no brainer.