Using High Templar during attacks is a very good idea. There is a reason and i can tell it to you in 1 word. STORM! When you're attacking bring along some high templar. An even number would be nice, you'll see why later. When you attack just storm the enemy force and in no time, there we be nothing left of it except for some dead bodies. Now, Templar run out of energy after about 3 storms if they have the energy upgrade. Here is why you should bring an even number. After they run out of energy, just turn them into Archons. Archons are one of the strongest units in the game with a fast attack speed, 30 base damage and an insane amount of shields. Cept if they are EMPed, they're pretty much screwed so be careful.
You should also bring some Dark Archons along too but this is optional. Dark Archons can do Mealstrom, Feedback, and Mind Control which are very useful if used correctly. Especially Mind Control.

CreepColony's Opinion: Everyone hates high templars...especially zerg. Even though psi storm got nerfed in 1.08, it is still devastating.