That's like asking someone who looks physically fit what their secret is, and they tell you, "Diet and exercise."
Everyone knows this, but no one wants to put in the work.
If you are a professional speaker on a tour, someone with a lot of drama experience, or a salesperson with the same pitch every time, then it might make sense to do Gladwell's approach. For the rest of us, practice extensively but don't memorize start-to-finish.
It depends. There are some advantages to winging it: true spontaneity, an ability to have a more interactive presentation that is tuned to the rhythm and responses to the audience.
The oft-materialised risk is that it turns into a rambling incoherent mess.
In front of an unknown audience, the risk is too large, because if they don't feed you the right response, you are dead in the water. At least with a prepared talk you have something to fall back to.
If you know your audience, or if you are determined to engage it whatever it takes (school talks come into this category), a certain amount of off-the-cuffness works well, provided you are confident you know what you are talking about.