Presidential debates can be useful in some respects. Take the current cycle. Early on, Ben Carson and Donald Trump demonstrated that they are woefully uninformed in many domestic and foreign policy issues.
Christie has shown that he's an ardent proponent of the surveillance state. Kasich has shown that he's not up to the challenge of the culture wars.
However, presidential debates have serious limitations. Apart from immigration, I think Rick Perry would have made a good president. But because he went into the debates unprepared four years ago, he blew his chance. He thought he could wing it. He was wrong.
Jimmy Carter was smarter than Ronald Reagan. Yet Carter was one of our worst presidents while Reagan was one of our best presidents.
Reagan was manifestly over-the-hill in his first debate with Mondale. It was painful to see him struggling to maintain his train of thought and grope for words. Yet Reagan past his prime was vastly superior to Mondale in his prime.
Donald Trump has a very long paper trail. You don't need to watch him in a single debate to have an informed position on his qualifications–or lack thereof. His reputation precedes him. And, in fact, the things he's said and done before he decided to run for president are a more reliable guide.
The debate format lends itself to simplistic answers. Canned answers. Take the coed the military. For a Republican candidate to give a defensible answer in opposition to women in combat, or women in the Navy, he has to lay the groundwork. The general culture will instantly demonize him as a sexist bigot. It takes a lengthy explanation to put the correct answer in context. And that's not possible in debates with one-minute answers or 30-second responses.
Same thing with abortion. It's basically impossible to give an intelligent answer to that question in 60 seconds or less.
Debate preparation makes debaters sound scripted because that's what the format demands. A better metric is how candidates respond when they have more time to collect their thoughts and give detailed explanations.