Multiplayer/Co-Op & Frictional Games? NO!!!

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But that's the point, if you get tired of them you could sacrifice them, without getting game over.

Well I probably would have taken kinder to "Ico" if that had been an option... (just kidding folks, Ico is a great game. There's no denying though that Yorda isn't exactly the brightest NPC there is.)

True reason why many oppose the said feature is that it takes development time away from the Single Player.

Well that and that I guess it is very difficult to design a game or even a level that works equally well in Singleplayer as it does in Co-op. (Unless your name is Valve and you have as much time and money as you want, in which case you just create two separate campaigns.)
It would work in a shooter, where you just adjust the number of enemies accordingly, but how do you design a riddle so that it is possible to do alone, but the second player still doesn't feel useless?
I think one of the two play modes would always suffer.
Even in magicka, when you're playing alone it feels like you're not playing the game as it was meant to be played. First of all because the one-life-only mechanic, combined with the many, oftentimes unfair (but hilarious) ways to die indicate that the game expects there to be a second player who can always resurrect you. And secondly because all dialogues seem to adress multiple persons.