Radio ads need to run to a certain time. 30 seconds is 30 seconds not 31, not 29 - 30 seconds. The same for 15 seconders, 45ers and so on.
If I ever write a script for you, it will run to time.
Quite often clients will pack their script full of words and it runs long. I tell them it runs long. They say how THEY read it to time. I tell them that's not how it works and they respond with "can't the voiceover artist just read it faster?"
Yes, the voiceover artist CAN read it faster and the audio engineer CAN speed it up... but what benefit will it have?
The ONLY benefit: It'll get all the words in the script.
Speeding up the script will reduce the quality of the commercial - it'll sound sped up and it'll be too fast for the listener to take in and retain.
Word economy will help cut down the word count.
Another one from clients is "I read it to 30 seconds, so it's fine".
Welly, welly, well. Sure, they may have read it to 30 seconds but did they read it at a nice pace? At the same pace as the voiceover? Did they time correctly?
At the end of the day, as a writer, my job is to help a client run a good quality script. If you ever allow a low-quality product go to air, you don't care about A: your business or B: your client - depending on which side of the coin you're on.