If you work at a little bit, it will become useful even in level 20 content. I've got a HORC strength build void 4 monk who walks off the ship with a 65 with only Challenge and Crafted AC gear, slides into the low 70s with raid buffs. That is noticable in a lot of normal/hard content especially if you solo.
All of my pure monks put in a bit of work to have an AC option and I always find it usseful. Before the challenges, I would make sure to have an alternate gear set with guards and healing amp in place of the AC, but the challenge gear gives excellant guards and decent AC at the same time.