Like virtually any post-processing effect, grain can be used well or used poorly. Personally, I don't think there should be a toggle. It's an aesthetic choice made by the developer; the developer wants the grain, or the developer doesn't. Ideally, that choice should be deliberate and justified.
I get that games are interactive and scaleable, but you have to draw a delicate line. I feel that having the option to remove grain is akin to saying, "You know, I don't really like the color correction David O. Russell did on Three Kings. Let me just toggle that off before I watch it."
As others have pointed out, The Order uses grain to fantastic effect. You could take the grain out, and it'd still look great -- but if the other elements of its presentation weren't enough to cue you in, The Order obviously aims for a cinema-inspired look.
Unlike The Order, some games would downright suffer without it. Divisive as they may be, I think effects such as chromatic aberration and grain in ZombiU -- alongside its great lighting -- make it a fantastic-looking game. Take those effects out, you're left with something decidely average-looking, and far less atmospheric. If I may use some screenshots taken by Gaffer fabricated backlash:
It's alright to not like certain aesthetic choices, but I certainly don't think we should call for less options in terms of the developer's expression.