for twins you only need 1 or 2 points in IA (cant remember exactly) outside of the twins fight IA has a questionable use, it all depends on you either taking fire/frost dmg to use fire/frost ward or havin a discipline priest/healer with legendary mace
and just cuz u have either of those doesnt mean you would even get a shield from them, if they are for example just healing the tank all fight. speaking to them and asking them to shield you every now and then is an option though but dont expect them to do it
if you have a raidhealer with legendary mace then thats great though
also pallies sacred shield or w/e its called works aswell so again you have to ask to get it