Think of it as using the ChromeOS kernel (which suits the hardware configuration perfectly) and then running the Ubuntu userspace (which co-exists with the Chrome OS userspace).
Even if you don't use the Chrome OS userspace much, removing it won't save you enough to make it worth it.
It depends a bit on the model, but in general yes you can. I haven't done it on a Chromebook, but I've completely wiped out ChromeOS and ran Arch on an Asus Chromebox (using instructions meant for the very similar C300 Chromebook). Worked perfectly, great little machine. Only hiccup was I had to flash a custom bios.
http://www.cnet.com/how-to/how-to-run-both-chrome-os-and-ubu...