The PLEX system is one of the things that interests me about EVE, and for that game I think it is a perfect fit. EVE is INCREDIBLY economy based, the entire game is based around it. Your guild is not a guild, it is a corporation. There is industrial espionage, there are guarded convoys for moving materials to market. Tieing the currency to something with a real-world equivalent goes hand in hand with the rest of it.
Does that fit with FFXIV? Not so sure. There are some pros to the system for sure, the big one is that it cuts down on the more aggressive RMT. 1.0 managed to avoid these problems because they pretty much just sat in cedarwood and killed leve karakuls all day, but I remember in XI having to fight for claim with someone who would scream at me in chinese every time I succeeded. It adds more wide-spread competition, which means their corner of the market is smaller, and less will try to occupy it. I foresee that it would increase the number of people who make the real world money > gil transfer, but cut down on the amount of RMT farmers in the community. Maybe a worthwhile change, hard to say.
For me though, the reason that would bother me to see it in this game, is that it does tie a real-world equivalent to your in game currency. For EVE this is fine because a lot of the game is about playing with market factors and having that little extra bit of risk when you take your ship out adds to that. But for FFXIV, I feel it doesn't. The best gear in XIV is all untradeable, and that pattern will likely continue in 2.0. So what it means is that the few big-ticket items are really expensive comparatively. The $-gil comparison is only visible if you go looking on RMT sites, but with the introduction of a PLEX-like item it becomes very obvious. Would you be willing to spend the equivalent of $50 on that double-meld relic weapon? Money becomes more of a concern if there are fewer steps between it and the real world, and to me that ruins some of the "fantasy" in the game.
And it doesn't help that for a lot of us coming from XI, ANY form of RMT has a horrible stigma attached to it because we did interact with the farmers. They were everywhere. We remember the christmas gil sale and how it devastated the economy, and how it took years to recover from. In any case, I doubt SE will be doing anything like this for the launch, alternate payment models seem like something to mess with after the game has been out for a bit and the community more stable.