I was extremely lucky to catch the Localization Team's resident ahriman, Fernehalwes, in the brief 40 second period of free time he has while his work PC boots up.
He explained the following reasons that the team decided to go with the numerical numbering, versus the current Japanese style of -ga, -ra, etc.
First off, having a numbered spell makes it instantly recognizable and understandable. If you're a healer in the heat of battle and your party is dying, and you're looking over your cure spells and see "Curaja" - are you able to recognize what level that is? Is it before cura? After? Where does it fall next to curaga?
Secondly, there are a lot of new spells that are being introduced in FFXIV, that just would not make sense with suffixes like "ra" and "ja" added (Medica, for example). In a self-contained game, where you're sure "this is the full spell list" it's easy to branch out, and finalize the names in specific styles. But when it comes to the ever-evolving MMORPG, you never know when a new spell will be added after a level cap increase. What comes after "za"? Is it really possible that those will work with every spell name?
Thirdly, this is something that was done early on the previous FINAL FANTASY MMO, FFXI.
Hopefully the above makes it a bit clearer on the reasoning for the decision!