If you read a spoiler, you deserve halo 4 to be spoiled for you.
It’s easy to avoid them, and there’s no need to go dark.
Just stick to official sites and sites you trust, and don’t read forum posts that obviously have spoilers in.
343 always give clear warnings of campaign spoilers, I haven’t seen anything from campaign except the teaser and e3.
Of course there are those weird people who like to read spoilers!
On another note, every game seems to get leaked about a month before hand, why?