Because phase 2 TFB, threat isn't an issue, but you may as well have the damage reduction on someone, why not a healer. 2 people in your raid taking 95% of their usual damage is better than no people in your raid guarded.
Actually, in Phase 2 of TFB the threat *is* an issue. Healers will steal aggro far too easily in there if they're not guarded, because the tanks are doing minimal damage to the Terror even with their full rotations.
Additionally, if there's any aggro held over from the first phase (honestly not sure if there is), it'll all be on the healer because the tanks weren't able to hit the boss there.
I don't think anyone who's done the fight will dispute the need to guard the healers in the second phase of the Terror fight. It's guarding healers any other time that's at issue here.