If you had a chance to play an ad-supported version of World of Warcraft, rather than shelling out $15 per month, would you go for it? Well apparently that's the way Blizzard had originally intended to make the game.
Of course we all know that it didn't turn out that way, since we're still shelling out money every month to play. It seems that after much researching they discovered that it just wouldn't support the game the way they wanted to.
While I'm sure they would have still gotten a large player base, I doubt that it would have been the same success that it is today. The up-front cash that they receive from each player monthly is likely what fuels their content-creating engine. Still, it's interesting to hear how some of our favorite games were originally supposed to turn out.