You underestimate the power of her E. Her damage is through the roof. Most supports throw out all of their spells then wait while she's unique that she deals constant E damage when you have any type of poison on them. Her W can look into bushes and also acts as a path blocker. Her Q Acts as the main harass doing high AOE damage. Her E is for when they decide to all in you and your adc. You poison one of them and just unload as many E's as humanely possible.
Spell thief works wonders on Cass and now with the new passive she doesn't have to kill minions to stack. Her only way to stack other than time is poking which is doubled by the amount of people there are in the lane. You don't have to wait for 6 as in reality Cass does a ton of damage. When 6 comes it just guarantees kills.
Please read this with an open mind thank you :)