Mar 13th 2009 2:14AM Because...
Both Healing Stream and Mana Spring stack if there is more than one shaman in a party. Mana Spring did not stack prior to patch 3.0.2. So if u got 4 pallys u can have only 1 GBoW. If u have 4 shammys u can have 4 totems in 1 party :)
Mana Spring stacks with the Paladin’s Blessing of Wisdom.
And furthermore mana isnt a big issue anymore. At least for me as resto. So we can place at least 4 more water totems now to buff party (whoo) if they need it. Like cleanse all poison in 1 1secGCD and reaply mana buff in next 1secGCD.