I asked a similar question on the MNK forums, and I wanted the NIN response as well. I'm an 80 MNK Taru and I love the job. I've got no other high lvel jobs and NIN or SAM are teh real contenders.
When do you use SAM or MNK instead of your NIN? During what kind of events or activities do you find a strong difference in the play-style of one versus the others? Sadly, the playerbase seems to have put NIN in limbo since DNC, MNK, PLD or mage jobs squat so heavily on its playstyle. I hardly ever see anyone on NIN outside of campaign...
Please note, as a largely casual player, I'm not going to level and gear a job just for minor tweak in performance or playability here and there. I'll likely only invest in NIN or SAM if I can strongly differentiate it terms of what I can accomplish and contribute.
Feel free to be as general or specific as you like.