AMS is also a must have to protect heroes.
It's the only thing stopping really bad things like Storm Bolt, Shadow Strike, Entangling Roots, Frost Nova, Carrion, Shockwave, Breath of Fire, Rain of Fire, Blizzard, Flamestrike, Mana Burn and magic damage thingamagics from damaging your heroes. Other races don't have this ability on their heroes.
Now that we can coil our AMSed unit, AMS becomes even more vital, esp to the heroes. They dispel the AMS ? Cast again. AMS is cheap, and there's those 3 hero icons on the top left of you screen to make casting AMS easier.
Just remember to deactivate autocast curse - this is either to save mana, or to save the banshees and its surrounding units from things like the Mana Flare thingamagics.