Half a run per 9 over half a season is not that insignificant...It's like half a win.
Plus, the conversation isn't just Happ vs McCarthy. Someone else in the rotation will inevitably get hurt in some capacity. This adds to the currently non-existent depth. Jenkins, Nolin, Sanchez, whoever from AAA are all replacement level options at this point.
I wasn't one to go crazy about McCarthy going forward, but given the low acquisition cost, it was a move that would have helped. Who knows with Towers though. He might think Nuno > Norris or something.