Each encounter requires you do a set amount of DPS in a certain time frame in a controlled environment.
Here are my thoughts:
1. For this game to be at all balanced, a Horde raid must be able to do the same raid-wide DPS as an Alliance raid.
2. However, a single Alliance DPS class can always do more damage than a single Horde DPS class. If the encounter is threat-limited, Blessing of Salvation enables the extra damage. If the encounter is not threat-limited, Blessing of Wisdom, Might, and Kings allow for more DPS than the shaman totem equivalents.
The only way to reconcile statements 1 and 2 is for the Horde raid to get additional DPS from another source. The only possible source that a Horde raid can draw DPS from, which the Alliance cannot also tap, is shamans.
So there are three possibilities:
1. Shamans are expected to add some DPS.
2. Shaman Totems enable as much extra DPS as Paladin Blessings.
3. Blizzard has deliberately made an unbalanced game.
I'm pretty sure that we are all in agreement that Possibility #2 is wrong.
As for Possibility #3, I do not think that Blizzard--the makers of Starcraft--would think that making a game with two factions, where one faction is strictly better than the other faction, is a good idea.
So by a process of elimination, the only possibility left is that Blizzard expects shamans to add DPS. I do not see any other logical explanation for raid and class design.