I personally think the trade off needed to have co-op is quite reasonable. People are always saying how the plot was thin to begin with so any new plot holes two people might bring into the story won't be exactly ruining it. Its like saying shooting someone who has "3 days left to live" in the head will ruin the rest of his life.
Besides, we have all played the SP game right through and seen all the cutscenes. If they don't happen to be in the game it shouldnt matter as we've seen them all and no one plays the game for the cutscenes. They play it to control the player. Not to watch him. The reason to play the game will still be there.
Thats just my opinion so if your opinion is differant - all the power to you