Another speaker I heard recently ran out of time at the end of her presentation and exclaimed, "I didn't realize I had this much to say!"
Really? How could this have happened?
1. She didn't prepare
2. She didn't practice
3. She didn't time herself
To be fair, this was not a bad presentation; it was very informative and the speaker had great confidence and personality. It was not clear that she hadn't prepared, except when she got to the end and was disappointed not to be able to say more.
So just imagine if these two speakers HAD prepared. Imagine how much impact they could have made if they had really nailed it from beginning to end.
We can ALWAYS do better. We should never get complacent just because we speak a lot, have confidence and can whip out something in a few minutes. It's easy to start thinking we can phone it in when we are experienced and feel good about our skills.
Even if you're asked to speak off the cuff, you should be able to write down a point or two on a napkin to mentally ready yourself. Dig deep to find a story from your life to illustrate your points and you've got an opening. Recap your main idea and you've got a closing. You can do this in five minutes -- my group coaching clients do it all the time.
If you are NOT a last-minute replacement, then you have no excuse not to give the audience your best. It's never okay to wing it just because you think you can get away with it. You are doing them and yourself a disservice. We all have occasions when we are given a less-than-optimal amount of time to prepare. But please don't make it a habit. Phoning it in is not going to have a long-term positive effect on your audiences.
Always strive to do your best, give maximum value and be present for the audience. Why be just okay when you can be great?