I've always felt that the more senior/ highly rated presenters get the "bigger" shows e.g. beauty TSV launches so based on that QVC must rate her, though I do agree with you that these shows would generate good sales anyway due to the items being sold. I imagine QVC will analyse sales figures to assess the performance of presenters and presumably she reaches their required standard. Don't get me wrong, I'm not a fan, all this girlfriends talk, I'm such a girly girl, I don't cook etc really irritates me.
BTW when she was presenting the Yankee TSV she announced that she had kept the set she was given and would be giving it to her mother-in-law! Surely the aim of giving presenters samples is so that they can familiarise themselves with the product to help them when selling it,not so they can save on buying presents!