There is no such things as "traditional" baby's first foods. My wife still complains how her father gave her food when she was a baby - He put some of his food in his mouth, chewed it, took it out, and put it in my wife's mouth. She believes that this caused her to have tooth cavities and a bad teeth arrangement.
When raising our two kids, we sometimes relied on commercial baby foods (canned and dried) and usually just smashed and watered down our regular foods.
Your wife is right about the cavity thing. One of the dentists that we've seen for our kids gave us a phamplet saying that sharing utensils, straws, drinks, foods transfers the bacteria that cause cavities.