While a lot of things free can be exceptional, free games will never be the best. A lot of the times the 'monthly prescriptions' you pay to play an MMO is never really the money-maker for the company; it's all of the essentials either needed to play the game or the 'gift shop' items you can purchase from the game or even the conventions you hold and use the game as advertisement to get more people to show up. Really, games are just another form of active-marketing; get the customers involved and hooked into a business and make them become loyal.
I also find it rather interesting that pay-to-play games are highly addictive because most of us who are not hard-core gamers are thinking "Well, I am paying for the game, I might as well make my money go to good use and play the game".