I still don't like the idea that FFXIV would then have to rely on subscribers who are willing to pay for more then 1 subscription, in order for another player to purchase game time via in-game currency (which would basically mean, playing for free) while retaining the same revenue for the game. Even it isn't required to do so.

One of my biggest reason would be time versus money. If someone has a lot of free time, then (s)he would be able to purchase new content/items via ingame cash only. However, there are plenty of other people, who are playing the game on a casual level due to the fact that they don't have a lot of time to spend on a game such as this.

The first type of player wouldn't have to spend a single penny/cent/etc in order to play new content or get a certain item, while the other type of player is basically "forced" to purchase the content/item via real money, just so that (s)he can play the same content as others(/friends/linkshell members) at the same time.

Sure there might be people willing to do that in order to generate more in-game currency, but a lot of people won't. I mean, when given the option... Who would want to pay for a game, when you can play it for free instead? Would you purchase a loaf of bread when you can have the same one for free, but you just need to be in front of the store before it opens and hope that there will be something left when you reach the stands?