Actually Unix out of the box (unlike for example windows) does not have system level dns cache, people often setup things like bind, unbound, dnsmasq, nscd etc.

Systemd here tries to be the official standard system level caching.

I agree though, about your extra caching sentiment to a degree. Actually as long the caches are RFC complaint (e.g. they obey TIL - looking at you nscd & java) or should be fine.