It solves the problem of further bloating loot pools with unique sets. It doesn't matter how many classes you make that share the same gear in terms of "fairness". You have 8 party slots. 4 are dps that can be filled with any combination of jobs via the duty finder. You could end up with 4 str or 4 dex. What having monk and ninja would have helped avoid is when you have a premade group and get a drop that none of you can use, that drop for the week is completely wasted. There are aesthetic design choices I understand that keep mnk and drg separate, but there was absolutely no reason to not have nin and mnk share gear.