Problem is, HA isn't worth a card slot in a LG deck.
Oty, Scarab and whatnot are a different matter tho.
Some ppl even play it in conjunction with gravy shield to prevent the opponent's creatures from hitting them.
Although if you're primarily going to use it for that reason you probably want basilisk blood which will be far better at it. The only reason you would want heavy armor for the gravity shield protection benefit is if you were running Oty in the same deck, which begs the question of why are you HAing things under 6 hp instead of devouring them straight up?
Its just one of those cards that while technically 'balanced' stat wise with the other buff cards, is really not that useful.