Do great speakers write scripts or just "wing it" and ad-lib?

I got quite the surprise when I was working with a group of people and asked them to create a script, most fronted with some bullet points.

When asked to deliver their short presentation they struggled to keep it on point and succinct, their thoughts roamed, and often the simple beginning, middle, and end structure was unclear.

When I asked them why they had not written a word-for-word script they said that they were under the assumption that great speakers didn’t write a script and if at all, used only bullet points to deliver their talks.

So, there are multiple issues here I would like to address so you can understand the importance of script development.

1. I’ve seen many great speakers give an amazing talk without one cue card or set of notes to be seen. You might be fooled into believing that everything they are saying seamlessly and effortlessly is just off the cuff and they know their stuff so well that this is normal. It’s not what it appears and if a speaker is good, they will go to great lengths to make it look effortless. More often than not they have given that same talk publicly at least 20 times before the time you get to hear it, they have practiced making it look and sound like it’s the first time they are sharing it, this is the art of showmanship and comes from well thought out and practiced delivery.
2. They started with a bunch of ideas that had to be refined down into something coherent that fitted into the timeframe they were given to speak.
3. They now have a bunch of short presentations memorised in their head that they can combine into whatever size talk that they want to deliver.
4. They started small, wrote a script, refined the script, delivered the talk, went back to the script, and then refined it some more.
5. You cannot refine what you can’t replicate, this is why you write a script.

So now we have that out of the way, how do you do this script development stuff, well, let’s start with the truth, it takes time and effort, it can be a frustrating process, however, the more you do it, the easier it will become.

What are your thoughts on scriptwriting?