Of course it was convenient. She was the foil for Oz's own issues with wolf-dom. Realistically, she and Oz couldn't end up existing in the same space, one would dominate. This is one of the tropes used in many dramas and one of Joss' favorites. Faith did a similar thing for Buffy, Angel had Angelus, Willow had Amy and evil Willow (both vampiric and magic). It didn't bother me, becuase it moved Oz's character forward through his issues while touching on issues that couldn't realistically be resolved in the show (without it becoming the Oz show, anyway). While I agree with your criticism of the convention, I'm glad that the "second tier" characters of the show were explored as much as they were, even if it was through cliche.