Keep in mind tho, the more haste u go for, the more gimped the rest of ur stats will be. If u look at most of the haste gear the stats are typically haste and damage/heals only not that im complaining =) (this staff with its crit aswell is kinda an exception).
As of right now pre-2.4 The highest that a highest haste rating a healer could get is almost 300 rating or about 15.5%. But the stats on most of the cloth healing gear isnt that good. There isnt that much Spi or mana/5. So yeah you could have a 2.2 second Greater heal but youll just run out of mana faster.
Only class that I think really will benifit from spell haste are going to be mages, because im pretty sure I read that Shaste is going to effect global cooldowns in 2.4. Instant cast spells with a smaller global CD is going to be big for mages.
k so Haste doesn't make you go OOM faster as a healer. If you are spamming hasted heals, you obviously need the haste to keep your tank alive. Otherwise, it makes your heal go off faster and then you have more time before you have to heal again, so you potentially get into the 5 second rule much more often, and thus, regen more mana.
Spell haste seems to be one of the new greatly improved add on of WoLK. Lvl 80 gear is full of it and if as I suspect it helps me get my spells off faster as a Lock then it is a necessary item on the Hateful gear.