So while I was removing system.menu.css and system.messages.css, I decided to take a look at system.theme.css to see what that would mean for Zen's styles.
And it was amazing. All of the crufty bits in Zen's normalize.css is because of brain-dead rulesets from that one core stylesheet.
Yes, I had to move 2 of core's images into Zen. But by using Compass' inline-image() function, we can reduce the number of HTTP requests.
And moving those rulesets into Zen's SMACSS stylesheet organization really sheds some light onto why I was always uncomfortable with overriding that stylesheet. Its because the rulesets in system.theme.css belong to 3 different categories: base, modules and states. By moving those rulesets into the appropriate stylesheet, I had to create a global-states.scss file which had previously just been a comment styles.scss. I actually like this better; it gives me an better opportunity to explain its purpose.