First rule of VP selection: do no harm.
Regardless of McDonnell's merits, Romney has plenty of other prospective to pick from with comparable benefits, but who won't allow the Democrats to bring the "War on Women" into the Presidential race.
I can imagine the ads: "Bob McDonnell threatened to veto a bill that would have required transvaginal ultrasounds before an abortion. WAR ON WOMEN!!!!"
Sorry, but you really didn't pay much attention at all during that time did you? The law was tinkered with a little, but the law he signed is almost as bad.
He only threatened a veto, because the national media picked up on this and there was severe blow-back and so he backed down from what was wholehearted support beforehand.
Oh sure, I didn't pay any attention to a bill dealing with an issue I care deeply about in my own state.
From your perspective, any bill limiting abortion would have been "almost as bad."