i think he is saying that most teeth can be filled and the dentists would rather use crowns. i agree with that. sometimes a crown is wanted because otherwise the filling would most likely break or the tooth may be too sensitive to chew on with a large filling. in those cases, i think a crown would be ok because there isn't anything else to do but pull the tooth. but, yes, there is bacteria under all of them. a tooth needs to breathe and teeth have a natural of cleaning themselves, as well. when you place a crown, you cut off oxygen and saliva and starve the tooth of what it needs to take good care of itself.