No, spriests doesn't need another "burst on use" - they do already have high enough damage.
"already" means in your wotlk world?
He's referring to CATACLYSM right now, which will be a completely different game in terms of damage to health ratios, scaling factors, comps, buffs, gear and itemization.
And I agree, shadowpriests are lacking a damage cooldown (more so for PvE than PvP in current metagame) and PI fits right in.
Let's take a look at damage cooldowns come cataclysm:
Paladins: Avenging Wrath, Zealotry*
Rogues: Killing Spree, Adrenaline Rush, Blade Flurry, Vendetta*
Mages: Icy Veins, Combustion, Mirror Images, Water elemental, Time Warp*
Warriors: Death Wish, Recklessness, Bladestorm
Deathknights: Unbreakable Armor, Hysteria, Dancing rune weapon, Gargoyle, Empower Rune Weapon
Druids: Berserk, Starfall, Treants
Shamans: Feral spirit, Bloodlust Elemental mastery
Hunters: Rapid Fire, The Beast Within, (Readyness)
I probably forgot a few, but some of the classes have damage increasing cooldowns coming out the rear, while ours is very mediocre and acts as 2 functions where damage dealing is the secondary and mana restoration is the primary. Giving PI to shadowpriests would be a welcome change, as long as overall damage output is balanced around it, the same way warriors are with deathwish and paladins with wings.