From reading skill descriptions (my ROG is 27, so I can't pretend to know anything about Ninja), my assumption is that NIN is going to be very very useful primarily because of Trick Attack. It looks to be a hugely powerful move, and we'll see what the application is long-term.

As per NIN out-dpsing MNK in end-game... There are maybe 1 or 2 people that could tell you the viability of that, and they are probably passed out right now from playing for over 24 hours straight.

Let's wait and see, but it will continue to be an interesting topic.