They're free in the sense that you can play it as much as possible and reach the end game, but often, there are some things you HAVE to pay for(Mabinogis rebirth, that one games quest buying,etc) to really do much.
It depends on the game.
Free to play is free to play and the cash shop just provides more convenience among players.
It's not really a necessity to buy it but it does make things easier.
Not really most have a cash shop and others have a premium service. Since they are letting players play the basic game for free they have to find some way to off set that. But people still ***** about the game making money. Sometimes I wish F2P games would go P2P so we could get rid of all these moochers and whiners.