I use it anytime I see a Warlock or a Priest in Arena. One more GCD sucks but eating a 2k Shadowbolt is worse. Plus, all the dots an Spriest will put on you are all Shadow based, so using Shadoward will keep you from getting hit too hard initially, and will give you that few extra seconds to pop your Spellstone to wipe the debuffs off of you.
Also, if you're referring to Arena fights. Get Proximo. It's invaluable for making targetting and identifying the opposing team much easier.
Edit:
Proximo Linkie Edited, Oct 31st 2007 4:43pm by DarkcydeIntensity