Weakness Policy can be used to boost a weak pokemon to give it a tactical edge or a bulky pokemon (more common usage) to give it a free buff. What I like doing is a bulky Dragonite with the Multiscale ability. Dragonite can survive some of the heaviest of Super Effective hits and the free boost to it's attack means it can instantly retaliate with a devastating fury.
Another fun choice is Weakness Policy Carracosta. It takes the advantage given by Weakness Policy and couples it with its main supportive move, Shell Smash, to boost it's stats sky high making it deadly combination when used effectively.
What other competitive builds can really take advantage of Weakness Policy? Do you like using it or do you forgo it for more common item selections?