Here is why I prefer a 1h and shield, these are two different healing setups that drop in kara, one is a staff, other is shield and 1h weapon.
Nightstaff of the Everliving
33 stam
34 int
55 spirit
348 Healing
439 healing with talents and enchant
Shard of the Virtuous and Triptych Shield of the Ancients
46 stam
41 int (+12 with chant)
390 healing
477 healing with talents and enchant
14 mana regenerated every 5 seconds
4872 armor
Shaman need 2 stats heavily, Healing and Mana Regen, we do not have enough gear with spirit or Talents to make it desirable for us. Raw Added Healing makes our Earthen shield very affective and our Chain Heal very affected, Earthen shield in total gains 300% of our healing, and with talents like Purification and Improved Chain Heal Raw Healing Power becomes very important. Also mana regen is big for us, but this is a poor example, there are staffs with MP per 5 on them.
What is most important is the loss of either Healing or Spell damage, 1h and shield always provides more spell damage, or healing then a staff.
Also and this is just me, rolling on staffs with a clothy present is just messed up.
You see I also didn't mention that off hands generally provide more spell damage and healing to a clothy then a staff does, what staffs do have tho is greater amounts of Stats, stam and int on staffs tend to be a bit high, in comparison to a 1h and off hand. Shields help us become more defensive, well thats what many staffs do for clothies, rather then doubling there armor like we get from shields, they may increase there stam by 15-30, not a big deal, but when you dont have much else to increase for survival it helps, just like us wanting our armor even tho it is not much, it helps.
The only time you should ever take a staff in a raid is because, no one else needs it and it is a big upgrade for you, but if a clothy needs it, be a good guildy and let them have it, this may help them do better when they are doing there dailies or pvp and help them improve themselves outside of a raid to be better in raids.