content: Not that girls with tongue piercings are better at BJs, but that the actual jewelry attached supposedly stimulates the underside of the penis and makes it more pleasurable for men.  I can't speak from experience if that's true or not.  But like you said, hygienically, it can't be good to have a stinky belly button in your mouth to collect food, sugars, etc.  It probably feels like a blood boil on the tongue.