The freedom and power to control what's yours. A chance to explore, to learn more about the system you're using and possibly adapt it to fit your needs. IMHO giving users that chance is very important: how many experienced developers started out as power users - who also were originally only users? By taking away these chances, keeping regular users relatively oblivious, fewer of them will want to go through the increasing hassle of "becoming a developer"; it becomes an abrupt decision instead of the continuum of knowledge it once was. This divide between "users" and "developers/administrators" only makes it harder for users to cross that gap, and takes control away from them.
> If you want to fiddle with hardware yourself, you get a Raspberry Pi [...] These devices are open in ways Personal Computers never were, having everything from schematics to JTAG pins available.
Funny you mention the RPi, as it's nowhere close to being as open as the PC/AT was.