If you have separate partitions, would you really want user data to go to the system partition? Or a third partition?
Do you find having more places that user programs can write a benefit?
The fact that nobody does that is pretty much a consequence of the difficulty of coordinating multiple projects that do not have a common authority, not because it is a bad idea.
Maybe the reason no one does that is simply that no one shares your preference.
- easier to cleanup/wipe without risking deleting works/personnal files
- backup solution doesn't have to have a town of entries in an ignore/exclude file
- same as above for syncing software
- tier storage separation possibility
- disk space allocation separation depending on data vs volatile stuff
The fact that nobody does that is pretty much a consequence of the difficulty of coordinating multiple projects that do not have a common authority, not because it is a bad idea.