I can see why Jetpack has a custom CSS feature. Users of wordpress.com cannot create child themes. But that limitation does not exist on self-hosted sites.

I understand there are different situations but, in general, the way I see it is that if someone is savvy enough to write custom CSS then they are savvy enough to create a basic child theme.

If you have X amount of time available, why not use it to create a guide and example child theme rather than to develop an arguably unnecessary theme feature? It's better to train people than to encouraging shortcuts.