I would be in favor of the Flex Pass it it acted as a "Buddy Pass" with a Registration & Delivery System outside of the game world.

If it were an in-game trade-able (for gil) item, I do not see how it would limit RMT. In fact I believe it would encourage more RMT. Here is my reasoning:

I would basically have 3 options to play the game:

1) Pay my monthly fee of $10 or $15 USD.

2) Farm enough gil in-game to buy a Flex Pass: Price unknown but let's say 150,000 gil (could be 1-2 mil though)

3) Pay an RMT $10.00 USD for 1 mil gil, buy Flex Pass for 150,000 and still have 850,000 gil left over.
3a) or Pay RMT $5.00 USD for 500,000 gil and still have 350,000 gil left over.

Now if the Flex Passes start selling for 1-2 mil gil, yes the RMT's would have to work harder for less profit but they would adjust and everyone going for option 2 (farming gil) would suffer from inflation.

Bottom line for me though is this: I just don't like the idea of Purchasing In-Game money with IRL Money (which is what "sellers" of the Flex Pass would be doing) and is why I stay away from supposed "Free-to-Play" games and "Cash Shop" games and I would stay away from FFXIV if it takes that route also.

I'm sorry if "I don't like it" is not a good enough reason for anyone else but it's good enough for me.