Who actually does that this day and age?

I do. Helps massively when seeding virtual machines using auto-install scripts.

Same. It's faster and feels respectful to not keeping hitting freely provided repos for the same packages over and over. Someone has to be paying for the bandwidth.

Thinking about it, that's not a good argument for http being the default. If you're seeding VMs, you're already in control of the config. Using a dedicated repository proxy (apt-cacher-ng? sonatype nexus? pulp? something?) might even perform better than a generic caching proxy.

Yes, I use acng. I still consider it a caching proxy, even if it's an application-specific one.

Developers

So a minuscule part of the install base that can absolutely alter system configuration to use a local mirror.

Not something that should be the default, exposing everyone to the dangers.