IMO this is a tank ring cause the high ac / avoid more needed by a tank class but some people like to be stupid and those who dont really need ac/avoidance just want it for the amnt n jack from people who actually put it TO USE
So the fact that this ring has a caster focus doesnt even enter into your consideration? High ac is a tank concern, yes. High avoidance helps every class.
And since casters would be the ones actually putting the focus to use I would think your own arguement actually makes this less of a tank ring than a hybrid or caster ring.
As of next patch, Avariciousness of the Ikaav (20% beneficial mana preservation) is being added to it. Should make it even that much more valuable than it already was.