If some classes are overpowered in parties the game becomes too easy for everyone.

I do understand that classes like Ninja and ARC SHOULD do a bit more DPS than other classes because those players PAY huge amount of gil for every single fight.

There's a difference between class variety and total lack of game design. I think the game is heading to the right direction but naturally it'll probably take a few years to get all the stats working like they should.