Obviously with a fixed roster like 5-on/4-off you could take 5 days holiday and get 13 days off.
I once worked on this type roster, and it is not always the case. If you worked day 1-4 as OFF, day 5-9 ON, and 10-13 OFF then you could take that block of 13 as holiday with 5 days holiday. Now you work 1-5 ON, 6-9 OFF, 10-14 ON you now needed 10days holiday for 14 days off. It was a real pain and they gave no flexibility. They would not change the pattern. For those who wanted to know their days OFF 6 months in advance this was necessary, but there was a penalty. For those who didn't care, they had no choice.
What I did find with a random roster, in a sympathetic airline, was it was easy to request 1-2 days off for special events; in the rigid roster system they did not allow this; no flexibility. Real pain the bum and in 21st century 24/7 working one needs a flexible sympathetic attitude.