Consistent subs is what you would be looking for. In cases where the game does not automatically renew, which I believe is true for WoW, what players actually do with the time is irrelevant, what matters is whether they buy consistent subscriptions, and going by the many 6-month sub promotions I doubt Blizzard is likely to get many people that stop playing. Afterall, you get a 6-month sub, you play maybe a month or two, then you play something else, then finally you return when a new patch rolls around, and coincidentally right around that time you get another 6-month sub promotion.