Except what makes them "hardcore" and "casual"? That's the key issue here. What makes anyone fit into either group?
Playing to win isn't it, because there are those that play with a lot of skill but only like to goof around (for instance, some griefers. I've seen some people play like they were some sort of super-ninja, only to throw the match for the pure fun of it).
Playing for fun isn't either, because there are people that like to play games, not for the enjoyment of it, but simply because they want to either say truthfully they played it, or they want to speedrun it, or they're completionists... Does not enjoying it make hem less hardcore and more casual? And yes, people like this do exist. Completionists are a sort of OCD, and speedrunners, while they might enjoy the game, aren't doing it for the sake of playing but for showing their skill at the game.
I just can't find any real point to "hardcore" and "casual" labels, even if they are used in only the lightest of terms.