The other reason is that it's hard to tell who is who if everyone is wearing helmets, so even something like Band of Brothers which put a lot of emphasis on accuracy had to remove helmets in combat zones some of the time.

I never played a Halo game, but I have read the novelisation of the first game... and it was a rare novelisation that got someone good to write it. They expanded on things fairly well, there were flashbacks to first contact with the Covenant, John has flashbacks to his training about ringing a bell, flashbacks to Halsey, there are plots about the Covenant reacting to John's rampage, and a particular grunt that decides they don't want to be cannon fodder.

Lots of these things could be done cheaply.