Sky Walker can be useful after Lunar Leap, when falling down from a highground, for drifting in a duel or just for travel a lot. Its realy intresting and funy to use, I see ppl use it, but I still don't find it so effective. Moonwalker has the same bonus but it gives it when Io is healing. I see no reason for it, because when you are healing you wanna go safe, not drifting. It can't be useful for a traveller skill, because her heal needs some aim and have a limited range, so can not be channelled all the time, while shooting is passible always if you have ammo. Drifting while attack an enemy can be useful, but we wanna be close to a corner while healing, don't dancing in front of the enemies. This is why Sky Walker is a much better pick than Moonwalker.
I think Moonwalker could be also good, if its lvl1 bonus would be buffed from 50% to 70%. So its max level (-110% gravity) would fly up her slowly while she is healing and garants realy good air controll for dodge the enemy attacks.