Orcs cannot be portrayed as unintelligent because such a race would not be able to survive on its own. They would simply be slaughtered like animals.
Goblins are also associated with orcish pharmacology; for example, in Age of Wonders, they specialize in poisons (which makes sense, as when you're so small, how else can you take down a larger opponent?). So, it's a great alliance.
Without the support of pharmacology, which enhances speed and agility, shamans wouldn't be able to fight on equal footing with elven dancers or beastmasters because they don't have the strength of male orcs, the agility, or the years of combat training that elves possess. Shamans only have ritual daggers and potions; without them, any militiaman could kill a shaman like a cook with a knife.