There's a ton of harm, that's a ton more effort in server, game, and level design to accommodate a potential 128 players. All for a best case scenario of never even noticing it because the maps are so big and the players are spread out, to the worst case scenario of half of the maps feeling like Metro.
128 players is just bigger numbers for numbers sake. If you want to see what a Battlefield game looks like with a ton of dudes go play Planetside 2 and realize how bad of an idea that is.
It's garbage because Planetside 2 isn't built to actually support 128 players. There is no territory control, no metagame and most of the large scale battles are reduced to two huge tank balls fighting each other. I'd argue that a game can create a framework where high player count can be properly used but I don't think that DICE are actually good enough to do it. I'd take 64 players over a bad attempt at 128.