Faster GCD would ruin positional DPSs like monk and drg and it would tame the mechanics of the fight. The more you burden the player with skill spamming the less attention he's going to give to fights mechanics.
Wow has faster GCD but all I remember as DPS is parking on the feet of some boss and furiously spamming skills to top that DPS meter. The same with healer, parking on some corner and playing whack a mole with healbot.