The Tux3 filesystem returns

full_page_writes doesn't mean that full pages are written all the time though - it means that the first time a page is touched after a checkpoint it will be logged entirely. If you have write intensive workloads the system should be configured in a way it doesn't checkpoint permanently...
The reason its on by default is that its important for consistency in the face of OS/HW crashes. Postgres' pages are normally bigger than the disks sectors and you could get into problems if e.g. one half of a page was written but the other was not and the WAL record only contains information about the one of them. As recovery always starts at a checkpoint its enough to log the page fully once after each checkpoint.