[Jei]

Unfortunately even PLD you will need /nin too at end game. Altho it's situational.

HNM
For Mnk you'll be using /Whm. You'll boost MND for Boostx16 Chi Blast. Which can deals upward 1k damage. You'll dealing damage comparable to Blm here.
For PLD, /War for normal HNM/Gods. /Nin for Grand Dragons like Jormy. Not every LS can do grand dragons so you might not have to worry about /nin too much.

Merrits.
Here's the thing. In a merit PT you want to kill as many as you can with as little downtime as possible. Cutting the MP usage is really a huge factor for a successful merit PT. And Utsusemi is what will make that happen.
Not only Mnk/nin is prefered here, even PLD are prefered to use /nin too.

Of course you can stick to War if you want but you really are making things harder for everyone. This is for every DD too. Sam/nin, War/nin, Mnk/nin, Rng/nin, Drg/nin and so on. While /war can deal more damage, damage is usually not a problem in merit PT because you're only chaining VT and low IT, plus your merit will help with damage even further.

Anyway, give nin a try. It's a fun job. If you really is determine not to level Nin at all IMO the only job that will never need /nin are Blm and summoner. Yea even Rdm Whm and Brd will benefit from /nin too.