i did and there's nothing relevant.it only sais how the commander met the hero and they fought together on the battlefield ever since.play heroes chronicles and u'll se that shamans are wise people who [some of them] become advisors of barbarians [in the world tree and the fiery moon when tarnum was a barbarian, he had 2 shamans at his side; in the revolt of the beastmasters, though, when tarnum is a beastmaster there is no mention about shamans at all]
another reason: barbarians are the only people who believe in the Ancestors. Shamans pray to the Ancestors and call for their help too.