well, in all honesty, I don't see any reason to use /NIN in exp parties. I mean, sure, you get elemental debuffs, only, most of your spells are not magic spells, they are physical spells <.<; And those that are magical are mainly used to MB with <.<;
Some Blu spells have effects based off TP, which makes /NIN viable to me.
I don't think it counts unless you use Chain Affinity, sadly. Atleast, I haven't noticed any difference between 0, 50, 100, 150, 200, 250 and 300% TP.
Besides, with the resent "tp nerf" you won't really build TP any faster with Dual Wield, especially considering you will probably nuke most of the time unless you're soloing. Either way, I'd say WHM, BLM, WAR, THF and DRK are much better subs that NIN.
WHM - Divine Seal, more MP, MND, -na etc. Very good for party situations.
BLM - Conserve MP, more MP, INT. Good post-30 for extra nukes
WAR - higher attack, STR, DEX etc. and some nifty job abilities. Also good for tanking.
THF - good post-30 since Blue Magic stacks with Sneak Attack.
DRK - same as WAR, only you also get more MP.
I honestly can't see any real benefits from using NIN sub other then shadows, but that's a logn recst time and cast time, so imo, not worth it =/