The fraction is obviously significant to the people trying to make sure airplanes are fully utilized, so this seems like a word game to me. People miss flights all the time.

A brief look for stats doesn't turn up hard data, but TechCrunch has it at ~5%:

https://techcrunch.com/2017/04/11/overbooking/

Someone who worked in ticketing for two airlines says it's 2% for flights to hubs, and 5-8% for flights from hubs, because of missed connections:

https://www.quora.com/What-percentage-of-people-who-check-in...

According to IATA, a good year for airline industry profits is circa 5%:

http://www.iata.org/pressroom/pr/Pages/2016-12-08-01.aspx

So even if we take the lowest fraction, 2%, filling those seats represents 40% of the profit margin. That sounds pretty significant to me.