He would have been the perfect answer to all the GOPe disingenuously claiming that Trump isn’t conservative enough by their standards, he would have assured a strong turnout of the base, he would have been a great VP attacker and debater, and he would be seen as countering some of Trump’s perceived weaknesses—such as no experience in DC government, a sometimes shaky understanding of constitutional nuances, and/or Establishment smoothness.
Bringing in another state is usually overemphasized and not as important as contributing to a compelling and complementary ticket overall.
You've made some, in my mind, compelling arguments in favor of a Trump/Cruz ticket ... the only real question is whether that is possible now. Lots of bad blood between them ...