You can use one for a DPS group and one for a healer group though oom healers is not a nice thing.
So priority of needs?
Shaman (resto/elemental)
Paladin(prot/holy)/Priest/Warlock/resto druid
Mage(Fire over frost)
Druid(balance)
shadow priest
hunter
My argument is that shamans give the most buffs to a shadow priest and therefore deserve the shadow priest. The next set all benefit around the same in a raid warlocks do get more out of a shadow priest then any other class now that spiritual attunement got fixed. Dps druids and mages are less important as they are good dps but the druid is slightly more important then the mage because it would boost shadow dps a little however a mage with infinite mana is mroe deadly
. I place shadow priest before hunter because it would double the group regen however you would lose the regen in another group. Hunters are mana dependant however they provide better melee buffs and benefit a caster group by next to nothing.