They only did co-op because they needed to a new "gimmick" to make it seem fresh

They did Naval combat for two games (three if you include Rouge) and they couldn't do it again since it wasn't very AC like so the best thing they thought of was co-op

I honestly think this is the main reason why the game is such a mess, every main AC game so far has been solid, they add co-op into this instalment and it ends up being crap...that's not a coincidence.

It's a perfect example of how co-op is not such an easy thing to add in a previously built up single player games as you end up spending too many resources and man power on the feature instead of other, better features, improving current features like the story/stealth mechanics and fixing the bugs/glitches/frame rate issues.

I heard removing the disc from the console improves FPS significantly

Agree

They clearly said that if you are playing "single player" disconnecting from the internet may improve framerates. They are trying to offer suggestions that may help people's experience while playing single player, but that doesn't mean they are saying to ignore the co-op.

I do agree that they should have spent more time making sure the console versions ran better, but aside from occasional glitches with the NPCs I haven't had any issues with the PC version. It runs solidly for me.

Well the graphics and content is really great in AC:U. But the performance is what ruins it. And it's a shame, so much work has went to waste because of bad optimization.

Everyone can make food look good on plate, but making it taste right for everyone is the difficult thing.