It kind of depends. Romaji is probably understandable by anyone capable of playing FFXIV.

If you're typing simple short sentences, then being able to type in hiragana would probably be appreciated. With the IME you still type in romaji, but when you hit [space] it will convert your text into hiragana.

But if you're really trying to have a conversation, then even all-hiragana writing gets pretty hard to understand.
Itwouldbeliketypingoutreallylongsentenceswithoutusinganyspacestoseparatewords,
soyouhavetoreallystareatitandtrytofigureoutwhatisbeingsaid.

In that case, maybe Google translation is best?