Check out this short thread discussing DK+shaman vs Pally+Moonkins:

[Moonkin Aura + Heart of the Crusader] vs [Ebon Plaguebringer + Totem of Wrath]

IMO if you're running 3 DPS shamans then you're better off with a DK. If you're running 3 moonkins then you're better off with a Pally. A Pally doesn't really bring much extra benefit to a shaman group (Heart of the Crusader does not stack with wrath totem) whereas Ebon Plaguebringer is very useful to shamans.. Similarly, a DK doesn't really bring much extra benefit to moonkins (they already have Earth and Moon for the 13% damage) but a pally does (moonkins can benefit from Heart of the Crusader)