NIN doesn't share gear with MNK.. Not sure what planet you guys are on.
Unless you are talking about pre-2.4 gear (such as Soldiery gear), which was only done so that they didn't need to release a whole chain of NIN gear for those tiers.

i110 Crafted, Poetics, and Dreadwyrm are all unique left sides (xxx of Scouting).. Their only shared slots are right side (with BRD) for aiming Jewellry.

As far as crafters go - you now only need 2 sets (one for DoH & DoL) due to the Artisan left sides - which is a huge increase in Armory Chest space...
Weapons however are a major issue, along with PvP sets (no Acc) if you go that route, on top of that - some people have multiple sets (or at least pieces) depending on Acc cap for certain fights. I for one have a lower acc set for my main, as well as one with T13 acc.

It is definitely a worry when you throw another 3 battle classes on top of all of this.