Justicar seems terrible in control Warrior. If you're about to literally die turn 7 or 8 to face hunter, it's bad because you won't get to activate it twice so it can be a Priestess of Elune. If you're going to die turn 10 or 11 to a mechmage that's building board position, it's bad because its' a 6/3, so at least one more minion will survive than if you had played a normal sized creature, and that minion will be hitting you for 3 every turn while you heal for 2 extra every turn.
If you play it against a deck that is going to go for a looong, grindy game, it would be good if you had a 50 card deck, but you don't. It's really easy to deck out as control warrior, and taking out your 6/7 for a 6/3 just means you'll end the game with no deck, no board, 50 armor, and an opponent with a 4/4, in all likelihood.