Wanted to add my input on this topic since it's been something I've been patiently waiting on since Reach dropped almost a full year ago. I started the Halo titles way back when with the original Combat Evolved, and it was a campaign experience that I played through entirely with my father. This became something of a tradition as the games released; a new Halo would drop, we'd pick it up, and then we'd play through the title in split screen cooperative. We did this all the way through 4.
My dad is incapable of playing games anymore because of issues that he has with his hands, but the Halo titles have become a way to interact with my brother, who suffers from severe anxiety disorders. We played through every single Halo the same way my dad and I had, and it slowly became a tradition of our own. Now, neither of us have working 360 consoles, and the XBox One was unfortunately a system we had to pass on. But I do have an engineering rig that I run steam through, and he has, time and time again, expressed interest in replaying these titles that very same way.
I cannot speak for anyone else, but each Halo game has been extremely beneficial in providing common grounds for the people in my family, and every time I see a new game drop I get hopeful again that it'll finally be implemented. Unfortunately, each time, that hasn't been the case. At least not yet.
I really hope that 343 sees the importance of this feature, regardless of the device it's played on. It's been a staple of the franchise ever since its inception and, for people like myself, it's bizarre to see it just not implemented at all. It's been the driving force behind all of my interactions with the series, and it just isn't there. In any capacity.