FWIW, I don't use GWMT, nor google ads.
The only Google IP's allowed are 66.249.64-95 and ONLY from their bots. No previews. (nor are these restrictions set upon Google alone, it's the same for Yahoo and Bing).
Should some widget user land upon content of my pages in a SERP (which likely cannot be found any other place on the WWW), and chose to not view because a preview in unavailable?
Too bad, so sad.
Content, content, and more content. . . .