Actually, it’s kind of presumptuous, IMHO. Get to first base before running to third base.
There's a little bit of that, but I think overriding that is the idea that the candidate is not afraid to appeal to the whole electorate instead of staying comfortable among like-minded folks. It makes both candidates seem stronger for the general election -- assuming they're not fringe candidates, which in this case they're not -- at least Mandel is the current front-runner on the R side.