My preference would be that Trump would focus on other lines of attack regarding Cruz. It’s not like there aren’t other criticisms that could be made (e.g. wishy washy on immigration, TPP). And yet strategically it might make sense.
Whatever the truth is regarding Cruz’s NBC status, Trump didn’t create these questions, but he is exploiting them. That may seem dastardly to some, but most who feel that way are probably die hard Trump haters would never vote for Trump even if he focused on more direct criticisms of Cruz such as immigration and TPP. So, talking about it does raise doubt in people’s minds, and it is a valid question, even if Trump is raising it for an ulterior motive. And until Cruz garners up the courage to come out swinging against Trump, this is what he’ll get from him - indirect attacks on Cruz that sow the seeds of doubt in people’s minds.
Just a few months ago, Trump said there was no issue with Cruz’s citizenship. Now he’s blowing it all out of proportion. He would sell his own mother down the river to score political points, just like all his fellow Democrats.