We had a big thread back in beta about this. They told us it made the game more simple because the spells go up in varying power thus I II and III(which is insulting since no other language does this) then when we explained how they don't vary in power at all as much as being totally different spells, the localization team decided to stop responding, probably because if the point was to make it simple but people would still have to read their skills to see they are different spells instead of just Fire 1, Fire 2, Fire 3, the same spell in variations. Then there was no point in not using the other suffixes. Another part of the explanation was that they couldn't have uniform suffixes for spell effects, then going on to say that it'd get messy to keep adding suffixes and determine which gets what suffixes.

Yet the other localization teams went the other way and saw no problems with it. Because I totally want to see Fire IV , Fire V, down the line. =/ The answer just sounded lazy and like an excuse to me, but I doubt they'll change it.