Another thing is that people say it would ruin the story line because bethesda would have to reduce the amount quest interaction(like in fallout the world around you stops) making it hard for players to do quest. Thats an easy fix, just make it like skyrim and have it have a real time player interaction interface.
People say that it would be hard for players to open there inventory because time stops around you well the inventory system works fine in other games like dead island and borderlands.
Some even say that it would reduce the amount cinematic clips. Well co-op works fine in resident evil and i seem to recall that game having lots of cinematic clips when it was single player and now there are more clips since they brought in co-op.
Now i think it would be good to have co-op because that way you could do more stuff(such as role play or it would improve trading) or you could do 2 man(or women) sniper/scout/caravan/soldier/raider/etc. squad. It would have a lot more memorable experiences. i remember playing borderlands and being super fun but i also remember playing co-op for the first time and having a lot more fun because we were running from a phycho. Fallout is fun when see your first deathclaw and crap your pants because all you have is a 10mm pistol but it would be funner if you and a friend run up to it and punch the crap out of his legs not realizing that you only have 2 seconds to live(please if you think fallout is dumb with co-op then please tell me that doesn't fun at all).
I think that fallout would have a more realistic feel to it because your team mate isn't just an ai with set tactics that get you caught while your trying to sneak past some enemies because if you had a real person as your companion then you would be able to have a better tactical advantage or even fun but some people are afraid of stuff like that.
I even read some where that co-op would be over powering because each player could have the best weapons/armor in the game but i don't think thats a problem because you can already do that in sp. please tell me your opinion on wether or not fallout should have co-op.