Mmm, this is a fun issue, actually. I played WoW some back in the day (I enjoyed it then, I do enjoy it now, I don't have the time for it now, I don't play. You will not stop me from enjoying it by telling me it is bad). Most of the time, I was running around by myself. The few times I DID group out of an instance, it felt like i leveled slower, I got less gear, and so on. It was actually a lot less fun, sometimes. Instances were different, they were actually pretty fun, never mind how retarded some people were. But since I spent most of my time alone, was it really multiplayer?
I think it was. What really was nice about it was the social aspect. I COULD have just played and not talked to anybody, but while I was playing, I was constantly talking to others online as well. It's sort of weird. I wasn't really "playing the game" with them in the sense I was killing enemies with them, but at the same time, it was more social than a single player game, and I'd say that's enough to make it multiplayer. I think that's pretty weak reasoning too, and it's certainly a debatable thing, but I felt it was multiplayer enough, and i certainly can see how this would get on somebody's nerves, because there was no need for interaction, it was just something I did because I enjoyed it.