Don't see the point... Personally.
Firstly, Espada is Spanish for "Sword", Japanese for "Ten Blades". Cero is Spanish for "Zero", Japanese for "Hollow Flash". Arrancar is Spanish for "To tear off", Japanese for "Ripped Mask". Basically, Hollow and their abilities, which includes the rank up to Espada, is orientated around the Spanish Language, as is Japanese for the Shinigami. If they were using Roman Numerals, they would effectively be Italian orientated. If that was the case, you wouldn't be called 'Arrancar' but the Italian equal. It just doesn't make sense. Everything about Espada is done in Spanish, and they use the same numbers we use in England and America.
Secondly, I just can't understand WHY they would use Roman Numerals? Yeah, it's something different, but it doesn't exactly serve a purpose other than to take up more room on the Espada's body parts.
Just my view.
Hiei