I used to be a dental hygienist.  Here's my theory:  Because of the antimicrobial properties of PG, the germs in the mouth are being killed.  It is the bacteria in the plaque that causes cavities and gum disease.  If you kill the bacteria, there is less decay and periodontal disease.  I, myself, have been 
vaping for 4 years all kinds of juices and no cavities and my gums are nice and pink. I also haven't seen reports on this forum of other vapors getting a lot of cavities.  I think this would be a real interesting area for scientists to study.  JMO