Me, I would miss the living, breathing world around me if Azeroth were offline. Even when I don't choose to group up or converse with others, I enjoy seeing other players going about their own business and watching guildchat burble across my chat box. I'll occasionally refresh a character that's been stranded on a realm my friends have moved away from, but I never last long before I drift back to a realm with friends.
Would you leap at the chance to play WoW in isolation with an offline mode? What if you couldn't switch back and forth, so that once you began playing offline, that character had to stay offline?