As Scipii I used them to great effect against elephants; elephants seem to get spooked very easily by incendiary pigs and war dogs.

War dogs are also good as kamikaze's to break enemy formations; the dog trainers gain experience from fighting, not the dogs themselves (or so I understand), so as long as the trainers survive, they're good, expendable, easily retrainable units.

War dogs can pretty much charge right UNDER the spears in a phalanx formation and bite right away at those evil barbarians. Even if they don't get past the spears, having them right in front of an infantry or cavalry charge to break up the formation a bit is a great help.