I finished the campaign not three minutes ago and I was initially bummed about how they handled cortana at the end.
As for arguing that Cortana is not an important character seems a bit rash. I thought she was the best character of the duo. Master Chief is 'just' a soldier, not saying very much other than being a bad ass.
Dr. Halsey isn't even addressed at the end, despite making an appearance early on, which could probably help cortana make a comeback. (Something I personally would like to see)