There was a time some while ago (as I blow mental dust off my college literature studies) when "passion" was not a word that carried wholly positive connotations. It came with implications of things like irrationality, moodiness, strong emotions, even violence.
Passion can be an asset if a person must maintain momentum in the face of hardships and competition. It can be the base for tremendously hard work (which is one reason I think employers like it). But, it could potentially be a liability too, if it drives a person to create conflict or inhibits teamwork. Depends on the role and the team.