The majority of WoW players has always been comprised of Casual players, players who don't play often but enjoy all the content and distractions available. MMO's are the a fancy playgrounds at the park, and Casuals are the kids who come to play on the weekends.
The subscription model is something that works against what Casual players are. It enforces a system that rewards players for spending more time playing the game every day over playing it casually or just on the weekends. A F2P MMO is a giant sandbox that lets you play when you want to without any real commitment.