Maybe they perform well at 39, but Guardian seems to be one of the squishiest tanks at 60s. I think the bracket is just too bursty to allow any kind of tankiness to be effective. (Except for disc priests, blizz thinks its an excellent idea to remove the cooldown from their shields)
It could work at 80s, it is a much more balanced bracket. Only MM has ungodly burst, which is getting nerfed this week.