I would assume spell haste would be pathetic for shadow priests as it only practically affects mind flay which is your lowest priority spell and not a big enough portion of your DPS.
(Just like crit only affects MB and SW
which again by themselves aren't a big enough portion of your DPS not to mention they only crit for 1.5X while itemz value crit rating as if crits are 2X and even for classes that can crit all spells for 2x 1 spell damage is generally > 1 crit rating.)