Well who would be teaching the other lessons that you would be teaching if not PT then @Pinkpetunias23 ?
If a FT teacher teaches 25 hours of French a week, and then the French teacher decides to go PT and only be paid for 20 lessons a week, then the school is going to need another French Teacher to teach the other 5 lessons, aren't they ?
However, I definitely wouldn't recommend 0.8 in Primary, as all you would save is 6 hours of teaching (the one nice bit of the job). Everyone would turn to you for all the report writing, all the planning, all the data, the concerts and assemblies, all the trips, and all the stuff and nonsense that takes up most of your hours. If this were about teaching, do 0.6 and have much firmer barriers about being PT and sharing your class.
In some jobs of course - bank or Post Office clerk, in retail, as a lifeguard or leisure centre assistant, cook, waiter, barperson, Operating Theatre Practitioner, and a million other people who do the work in front of them on the day - then 0.8 would be fine. But people who manage projects or work with clients on projects or academics and so forth, it wouldn't be a good move to be paid 4/5 of your salary to still end up doing 100% of your work.