You might as well go necrowagon, since you will sure as hell be defenseless tier 2. If you aim to use cripple, by then, your opponent will bust out the fairie dragons, dryads, walkers, priests, breakers, or the worst of them all... destroyers...
I don't think focusing a strategy on cripple is a great idea. You are better off going necrowagon and hitting at tier 2 before he gets dispell units. Think of cripple as a bonus, rather than of main use. ONLY use it on high DPS/food units like wyverns, tauren or use it when trying to kill their hero.