The Deflier like the Templar cannot do anything without energy. The Deflier takes a good amount of Minerals and Gas to upgrade it so that it can be a help but once you have done this it can really help you. Dark Swarm can be used so that any of your units in the orange gas clound are really hard to get hurt. Plauge brings down the life, not sheild> of a unit or building a lot, It cannot kill anything but attacking a Zerg base after using Plauge a couple of times all over that base should be an easy target. It is also fun to use on Bunkers or photon Cannons but especially Bunkers because give them some time and they will burn to the ground. The Defliers are best used as a first line of attacking on defense structures or large groups o units there for defense, After the Plauge has done it's damage go in for the kill with some strong units.