One argument is that there is now a $/gil exchange rate.
Say a pair of wedding bands costs $40 or 20 mill gil (for argument's sake). If RMT can offer 20 mill gil for less than $40, it makes more sense to buy the gil from RMT directly (fears of account compromise/banning etc notwithstanding) than to buy the bands from SE. There's no real moral argument against it as SE are ok with you effectively buying gil as long as it is from them.
Thriving illicit RMT causes economy hyperinflation. Anyone who played XI circa 2005-2008/9 knows all about this. Scorpion Harnesses went from 1 mill to 25 mill+ in the space of months. At that point you had to grind for months to get that much gil or, (as many people did) bot your own gil or buy from RMT otherwise you would be left behind.
Now there are some big differences that could mitigate this: There are no 'must have' pieces in XIV that are available through gil only. Sure, the min-maxers use penta-melded crafted i110s, but they are optimal, not necessary.
One thing this game does have though, that XI didn't is a housing crisis. On Balmung I believe (someone can correct me if I am wrong) there are no medium houses available, only large. There are no plans to introduce new housing wards or plots until after 3.0. Right now it's a race to get the gil for a large house before they are all gone too. Now someone can get that gil indirectly through the cash shop. A regular player has little to now chance to get a house now on Balmung for at least 6 mo+. That's an impact.
