The fairy gauge can feel rather pointless because it only interacts with one single ability. However, I wouldn't call Fey Union useless. It has its uses in savage and ultimate on the long strings of autoattacks that the bosses do in between mechanics, average tanks don't tend to mitigate autos so the constant HoT has a use there.

As for the fairy gauge, the easiest way they can fix how pointless it is is to return Fey Blessing to the fairy gauge but turn it into an AoE heal pulse that consumes gauge per pulse, like Fey Union but AoE, so there's at least another use for the gauge.