Totaly agree with all your Points
(maybe / on a 300gb+hd 20gb)
But two very possible errors are 1. Hard disk failure and 2. Problem sits in front of PC!.
If 1) Home is byebye
If 2) Could be for example delete a projekt ordner.. something like rm -rf on /home :-)
So on all helps for a system failure recovery i strongly urge for:
A regular Backup of /home,
and any other path that is important (/var/www... )
This should be at least on a second hard disk or dvds.
With disk space this cheap every serius user should have a extern XXX GB Hard Disk. No excuses for this.
Of course this is only the minimal! single user case.