Unlike guarding, defensive aggression has no useful purpose. Breeders should try to eliminate this type of aggression in their breeding programs. The dog may choose to use his teeth to prevent you from rolling him over. Even the most dominant dogs generally can be made more tractable through effective obedience training. Dominant dogs usually have stable nervous systems.
Terriers have high prey and re-directed aggression, especially pit bulls.
Two females, two males, or three total dogs in a household, unsupervised, provide an opportunity for this type of aggression. Obedience training can help, but unsupervised, the potential for fighting between the dogs, and subsequent injury is present. As above, effective obedience training can help, but the pair should not be unsupervised. In my book, a dog should never, ever bite or threaten a child. To me that is inappropriate aggression.