Psychic moves generally have bad coverage so using them if you have another STAB is generally not worth it. The only real use for it, IMO, is 2HKOing defensive Rotom-A with a Choice Band. Even at that, being locked into a Psychic move lets powerful Pokemon like Tyranitar and Metagross switch in for free.Iv'e read the Metagross analysis, and it doesn't answer this qiestion, so: should Metagross use Zen Headbutt, or it has bad coverage?
Edit: Evil Ragoo, I think this thread is what you want.
http://www.smogon.com/forums/showthread.php?t=67702