That's a question I've been asking since Xbox started releasing its exclusives to PC too. With the Xbox family of consoles being a "closed system" PC essentially now, the freedom when it comes to peripherals also has this kind of drawbacks. I think since Halo is an FPS, we should all gradually move to KBM, but I understand how a lot of players have been playing these games for almost 20 years now with a controller, so it certainly is a tough decision. My guess would be that the choice will be on the teams, as long as they have the same amount of players for each of the two input methods, so as to keep the match balanced. Then, the actual balance of the two will be brought into the spotlight, hopefully MCC offers a lot of feedback and data so that they are as close as possible.