The thermoses are definitely worthless but the manuscript pages do fill in some of the story which is pretty cool. Most of the manuscript pages are right in front of your face just for that reason and they normally foreshadow an even that is about to transpire. Some of them are hidden way out of the way for some weird reason. And keep in mind that you can't even find ALL the pages unless you play in Nightmare difficulty, which isn't unlocked until after you beat the game at least once.
I thought that both DLC were quality. I do remember The Signal being even more combat heavy than the main game though. So if you don't like the combat, which does get old after a while because it's the same thing over and over, you might want to avoid it. I'm guessing you got a used copy though that didn't come with the code? I still think it's worth the $7 or whatever it is. Too bad it will COMPLETELY fuck up the amount of points you have.