It can be nice to play cooperatively with one's significant other, or kid. Whether it's helping them, experiencing great game moments together, or just having fun, It's a welcome option. The original Baldur's Gate series had it, as did the Neverwinter Nights series. This didn't detract from the single player experience, it just opened up an additional way to have fun.
I really don't get the need for people to sneer at others who enjoy having that option in games they like.
The trouble with computers is that they do what you tell them, not what you want. -- D. Cohen