If we see N'tho and Usze only during online play, wouldn't that mess up the cinematics? During campaign, there will be missions where the Arbiter is fighting alongside you... now theres option for 1-4 people to be involved depending on the size of your 'team' online.
When you play offline, these two new Elites wouldn't be involved in the story at all.. how could that exist with one fixed plot?
It would seem odd if they were only around if you played online.
And it'd seem even more odd if they weren't mentioned in any cinematics.
What Luke Smith said in his announcement:
We mentioned that we’ve been having a blast. We have a lot of stories. Many of them would require that we explain some of the reasons replay will be a big factor, but not quite yet. Soon, we promise.