It's possible that advertising buys won't overlap completely, but as the targeting gets more specific, the targeted slice gets smaller and smaller ... to the point where it is questionable that the slice can have an electoral impact even if the lack of explicit rebuttal has a significant impact on the opinions of the targeted slice. Assertion-rebuttal is an effective rhetorical technique, but it is certainly not the only technique. Honestly, familiarity with the candidate's name seems to dominate everything else. Perhaps I'm underestimating the impact of unchallenged claims, but even in that case both sides are still on a level playing field.