I think choosing distros from slightly more diverse backgrounds makes sense. Ubuntu includes most Debianisms including dpkg / apt-get, CentOS includes most RedHatisms including rpm / yum, while openSUSE is a third way (that I know little about).
Replacing openSUSE with Debian wouldn't be sufficiently differentiated from Ubuntu, IMO.
The inclusion of something inheriting from Linux From Scratch might be warranted for didactic purposes - just for understanding what bits a Unix is made out of, fundamentally.
When I first clicked the link, I was hoping it would be something like Linux From Scratch. That however, would be a completely different class. More like math-for-future-mathematicians vs. math-for-future-engineers. I wonder if LFS could be adapted for EdX?
Replacing openSUSE with Debian wouldn't be sufficiently differentiated from Ubuntu, IMO.
The inclusion of something inheriting from Linux From Scratch might be warranted for didactic purposes - just for understanding what bits a Unix is made out of, fundamentally.