Trump can't compete with the members of the political class when it comes to policy. They are immersed in policy every day. It is their job. An outsider like Trump or Carson starts at a disadvantage, at least for appearance sake. The reality is that it really isn't significant or meaningful that Trump doesn't know what the nuclear triad is or what part would he prioritize. Once in the WH, Trump will be briefed by the bureaucrats and his own advisers every day on the issues. Being President is being more an executive and manager than it is being a policy wonk. In fact, being a policy wonk may be a disadvantage given the likes of Carter, Clinton, and Obama.
The real issue is judgment. Last night, for me, the issue that reflected where a candidate stands on judgment was the institution a no-fly zone in Syria. Cruz, Trump, and Rand Paul passed the test. Rubio, Graham, and the rest of them along with Hillary should make us all wary of their judgment.
So I guess the debates are best viewed as a think-on-your-feet-under-pressure test. And as a barometer that there haven’t been any big changes in the candidate’s general direction or major thematic points.