Ok. But why magebanes?
Magebanes have high magic despite being shadows, so they can withstand skills really well. So they can make use or wizardry to more extra tankiness against skills. To me its like enhancing what they do best.
Furthermore, Magebane ultimate ability is not to "deal" damage, but the skill and aura that provide very good supportive role. And because of that, people usually have to use skill to try to take him or her down, which in turn make the mag increase viable.
But not alot of people does that i think. I dont have a magebane, so i dont know also.