There is good reason for Co-op but the story must be coop related.
A chuck Norris or Rambo one man army action story doesn't work well. Some uses like QTE are disastrous. cut scene rather not. But handy for breaks. But doing squad based mission as story's. You are a team in SP with AI teammates. In coop with players with empty slot filled with AI.
Ghost Recon Advanced Warrior. We played GRAW 1 & 2 on 3 PC in a row each PPU powered I got this Black hawk down movie experience. Very Cool. It made the 3th guest gamer a Xbox360 console game buy a PC. Operation Flashpoint 2 Red River. Same thing. except the cut scene NPC / actor was very irritating.
Co-op works with team based story's or plots. The problem is AI if the story demands a team even for solo play. This is a very complex problem. As a AI designer you design your AI with specific gameplay in mind. But gamers are unpredictable play as it often not meant to be played. Like hanging back get the flow out of the game. Starting a spawn wave notice. Or rushes like Rambo so aren't moving as team. get shot in the back. Hinder visa versa. Or find exploit to do things the AI can't adapt to. And often the AI get the blame. But its large the player who play it as the designer had in mind. Good AI should also address strange behavior of gamers. But also the game flow could adopt to the gamers so to the AI. Problem this is to much for a game with tight deadlines so often not done. So to me the AI works fine a bit limited. But I know I need to take a regular action flow and move the AI as team. Real gamers. well we start OFP2RR and the pure COD guy was fast to action and dead. So I said this is not COD move as team.
It would be nice if the story scales well up from solo to team you then need not to use such advance AI.