I see what you're getting at. If encounters are so easy,nobody is really ever below 90% health
then it doesn't matter what your stats are. You probably should be DPS, honestly. Which raises the question, why are you not further in Heroic ToT if it's that easy? If it's a dps roadblock, you should definitely switch, since you already stated your healing isn't needed.
I'm starting to get them to let me DPS instead so they can 2 heal fights. DPS'd on Twins this week. We're not further along because the group started late, so we've been downing the first weeks in 496 gear. Now that the group is roughly i514, we're starting to push heroics.
But I still think that for a Shaman who is a fairly fresh 90 and working to solve mana issues does not want any Haste, that's something to look into later on. I for one, am used for the entire fight, and my throughput is often higher than even disc priests I raid with.
However this isn't always the case, many Rsham are used as a safety net, for the CD's, Mana Battery, and Mastery/smart heals for big damage spikes, in which case, Haste is preferred.
I think that's part of the problem. Our raid CDs are so good plus the mana battery that we don't have to heal as hard. But having a lot of absorbs in MoP with Disc and Holy Pals, making better use of the smart healing from HST and AA seems to synergize better. When I was at lower haste (871 and the 3764ish), I'd cast a heal on someone at 40% and see their bar fill before my Greater Healing Wave would hit in 1.4 seconds. It's not that we didn't need 3 heals, it's that my heals were so much slower than others.
So if a starting Sham is gearing up, I'd recommend Crit over Haste or Mastery. It's a huge Spirit saver. If you have a group that is constantly falling below the Mastery breakpoints, then go with Mastery. But I think once Healing Wave becomes mana neutral (positive with resurgence), then I really think Haste is a big way to go.