Crysta is simply a faux currency. When you buy it, you can then spend it on whatever services will accept it. I'd imagine that you can spend the Crysta you buy on FF14 and FF11 however you like. However, I will caution that the Crysta system in general suffers from the same failings as most corporate-sponsored faux currencies in that you can never buy $12.95 worth of Crysta. You buy $15 worth, and are left with a remainder that rolls over every month. Crysta then expires at a point 2 years after the Crysta point in question was bought, effectively barring you from stockpiling too much Crysta in advance.

Considering I have a VbV debit card, would I personally use Crysta? No. I'd lose money by being forced to forfeit whatever remaining Crysta is left over in my account. Would someone who would have to pay some sort of fee anyways to get a gamercard or a checking/debit account opened up benefit by using Crysta? Possibly. It's just situational.