Monks lose a lot and gain nothing offensive by subbing nin. DW doesnt affect h2h.
I didn't said they gain anything offensively. Mnk/nin is still a lot more viable in many BCNM and ENM than war/nin. With their evasion, and counter (when shadow are down), they can pretty much tank everything and do really good dmg. Just give them Faith H2H if you're not happy with their damage.
Unless we are talking about HNM tanking, Mnk/nin tank better than war/nin, and mnk/war will usually put bigger number than War/nin. Not having to change sub to always do pretty good is what make WAR so good imo, since it can adapt easily to any situations.