From Scott Remnant, who wrote Upstart when he worked at Canonical[1]:
"I don't think we ever disagreed enough for me to refuse patches from them - it was very much a case of they had different ideas to me, and couldn't contribute them because of the CLA
If the CLA wasn't there, we'd've just tugged and pulled and fought about patches like normal projects, and Upstart would have turned out much like systemd - I'd've been okay with that :)"
The CLA is absolutely why systemd exists. systemd didn't fork because there was no benefit to anyone in sharing code -- Upstart wouldn't take their patches. There are technical differences, but those could have been handled in code (like Scott says) if the CLA hadn't existed.